When you upgrade to HID or a bi-xenon projector system, one of the most important pieces of the puzzle is the wiring that ties everything together. We recently spent some time with the latest generation of the Morimoto MotoControl bi-xenon harness, and we wanted to break down exactly how it works so that when it comes time for installation there are no questions whatsoever. This harness is designed to provide smooth power to your HID ballasts and to a high-low system such as a bi-xenon projector, and once you understand how the connections are laid out, the whole install becomes remarkably straightforward.
In this guide we will walk through every connection on the harness, explain where each wire goes, and cover the grounding and mounting details that make the difference between a system that works flawlessly for years and one that fails prematurely. If you are shopping for the right HID lights relay harness for your build, this article should give you the confidence to install it yourself.
Why a Relay Harness Matters for HID and Bi-Xenon Systems
HID ballasts and bi-xenon solenoids draw more current at startup than a factory halogen circuit was ever designed to handle. Rather than pulling that load directly through your vehicle’s original wiring, a relay harness uses the factory headlight signal to trigger a relay that pulls fresh, stable power straight from the battery. That is exactly what the Morimoto MotoControl does. It provides smooth power to the HID ballasts and to the high-low system, ensuring the ballasts and solenoids receive proper voltage the moment they are triggered.
The best part is that everything is designed to be plug-and-play. Every connection on the harness is labeled, and there is a wiring diagram printed on the back of the packaging for reference. If you ever lose track of what plugs into what, the labels and the diagram will keep you on course. This is the same approach you will find on a quality HID ballast wiring harness, and it is what separates a properly engineered kit from a tangle of generic wires.
The Single Input Connection
Let’s start where the harness gets its signal. There is only one input connection on the entire harness. In the version we tested, that input was an H13 (also called a 9008). If your vehicle uses an H4 or a 9007 bulb, the connector will look a little different, but the concept is identical: there will always be three wires going into it, typically a blue or white and a brown.
This input plugs into the factory outlet that comes from your car, the same outlet that is currently plugged into your halogen light bulb. It provides the signal to the relay, telling it whether to perform the high beam or the low beam function. Because the vehicle’s own headlight switch is doing the triggering, your dash controls, high beam stalk, and flash-to-pass all continue to work exactly as they did from the factory.
Understanding the Output Grommets
From that single input, the harness splits into two sets of outputs, and they are color coded by grommet so you never mix them up. One set uses yellow rubber grommets, and the other set uses orange rubber grommets. Getting these straight is the key to a clean install.
Yellow Grommets to the Ballasts
The yellow rubber grommet outputs are for the ballasts. There are two of them, one per side of the vehicle. These carry the main power that fires up your HID bulbs, so they use the heavier gauge wiring on the harness.
One thing we noticed right away is that the harness is spaced out intentionally. One set of wires is significantly shorter than the other. That is by design, because the shorter set is meant to plug into the accessories (the ballast and the high beam) on the same side of the car where you plug in the input connection. You will want that input, and the shorter set of wires, on the side closer to the car battery. The longer set then reaches across to the opposite side. Planning this before you start routing anything saves a lot of headache.
Orange Grommets to the Bi-Xenon Solenoids
The orange rubber grommet outputs have a slightly thinner gauge wire and are clearly labeled high beam. These connect to the bi-xenon solenoids on your projectors or HID systems. The solenoid is the little mechanism that shifts the projector shield to switch between low and high beam, so it does not need as much current as the ballast, which is why the wire gauge is thinner.
Because both the ballast and the solenoid outputs are grouped per side, the wiring on each corner of the car stays tidy. If you are comparing options, this two-output design is what defines a true bi-xenon relay harness versus a simple single-beam HID harness. Only a bi-xenon-specific harness gives you those dedicated high beam outputs for the solenoids.
Powering the System from the Battery
After the input and the two sets of outputs, you are left with three other loose connections. Two of them are grounds (we will cover those in a moment), and the last one has an inline fuse. That fused loop is what powers the entire system.
This inline fuse loop connects to the positive battery terminal to feed 12 volts into the system. That way, when the relay is signaled by the factory input, it has the power on hand to supply to the ballasts and to those high beam solenoids. The inline fuse is your protection here, so make sure it is seated properly and that the connection at the battery terminal is secure. This battery feed is the heart of any quality HID relay harness, because it removes the load from the factory circuit entirely.
Grounding the Harness Correctly
The two remaining loose loops are grounds, and they come off the connections for the ballasts. We cannot stress this enough: it is extremely important that these be grounded directly to solid, bare metal. A poor ground is one of the most common reasons an HID system misbehaves.
Our recommendation is to look for a factory ground point where other accessories are already grounded. It is very common to find these on each side of the engine bay, and they make excellent grounding points for the harness as well. What you must avoid is grounding to a painted surface, a plastic surface, or anything less than perfect metal-to-metal contact. If the ground is compromised, the system may not work as it was designed. Take the extra minute to scuff away paint or find a clean factory bolt, and you will save yourself a lot of troubleshooting later.
Mounting the Relay the Right Way
The final point of interest is the relay itself. Here is some good news: the relay does not need to be mounted in any particular location, and it does not need its own ground. That gives you flexibility in a crowded engine bay. However, there is one detail that matters a great deal, and it is all about orientation.
When you mount the relay, position it so that the wires exit out of the bottom. This ensures that no moisture can get into the relay pack or the connector at its base and corrode it. If you leave the relay dangling inside the engine bay with the wires pointing up or sideways, water can seep down through the wires, penetrate the multiple rubber seals, and ruin the relay, potentially leaving you with a headlight out. Take a zip tie or two and secure it wires-down. It is a thirty second step that protects the whole system.
Final Thoughts on the MotoControl Harness
After going through it piece by piece, the takeaway is simple: this harness makes a potentially intimidating job very approachable. Everything is 100 percent plug-and-play. Follow the labels on the connectors themselves, follow the diagram on the harness packaging, keep the shorter wires on the battery side, ground to bare metal, and mount the relay wires-down. Do those things and you will have a clean, reliable install.
If you want to see the full lineup or grab a replacement, you can find the Morimoto MotoControl harness and match it to your bulb size. And as always, if there are any questions whatsoever during your install, reach out and we will help you get it sorted out.
Frequently Asked Questions
What does the Morimoto MotoControl bi-xenon relay harness do?
It provides smooth, stable power to HID ballasts and a high-low bi-xenon system, ensuring the ballast and solenoids receive proper voltage when triggered by the factory headlight signal.
How many input connections does the harness have?
There is only one input connection on the entire harness, which plugs into the factory outlet that normally powers the halogen bulb. It has three wires, typically blue or white and brown, that signal high or low beam function.
What are the yellow and orange grommet connections for?
The yellow rubber grommet outputs go to the HID ballasts, one per side. The orange rubber grommet outputs have thinner gauge wire and are labeled high beam, connecting to the bi-xenon solenoids on the projectors or HID systems.
Where should the ground wires be connected?
The two ground wires come off the ballast connections and must be grounded directly to solid, bare metal. Factory ground points already used by other accessories in the engine bay work well, but painted or plastic surfaces should be avoided.
How should the relay itself be mounted?
The relay does not need a specific mounting location or ground, but it should be mounted so the wires exit from the bottom. This prevents moisture from seeping into the relay pack or connector and causing corrosion or headlight failure.
Meet the Experts Behind The Retrofit Source
Tyler Jordan – Technical Writer
Tyler works directly with the TRS product R&D team, bringing over a decade of automotive aftermarket lighting experience to every article. An avid enthusiast with hundreds of personal installations under his belt, his content is built on hands-on knowledge from the workbench, not spec sheets.
Caleb Morgan – Marketing Manager
Caleb brings over a decade of automotive lighting industry experience and works with TRS customers daily on the front lines. With hundreds of personal lighting installations and a direct feedback loop from real buyers, his content addresses the questions and problems that actually matter.
