The Morimoto HD low-beam relay harness is designed for reliable power delivery to your HID ballast and easy connectivity, and after wiring dozens of these under the hood ourselves, we can tell you it is one of the most straightforward parts of any HID conversion. There are only six connections on the entire harness, and once you understand what each one does, there is really no doubt about how it goes together. In this guide we walk through every connection we tested and installed, sharing a few extra tips along the way so you can skip the troubleshooting and get your lights working right the first time.
Why an HID Relay Wire Harness Matters
When you convert to HID, your ballasts need a clean, steady source of 12 volts to fire and run consistently. Pulling that power through your factory headlight wiring is not ideal, because the OEM circuit was designed for halogen bulbs. That is exactly where the HID relay wire harness comes in. It uses the factory headlight signal only as a trigger, then draws the actual power directly from the battery through a pair of relays. The result is reliable, consistent power delivery straight to the ballasts.
Every connection on the harness is labeled, so it really should be pretty self-explanatory when you read it. Still, we like to go over each one in detail because the small details (like which side gets the OEM input and how the relays are oriented) make the difference between a clean install and a frustrating one. If you are new to the whole system and want to see how the harness fits into the bigger picture, our guide to HID projector retrofit kit components is a great companion read.
The Six Connections on the Harness
Before we start bolting anything down, here is the big-picture summary. The harness has only six total connections: one OEM input, one fused battery positive wire, two grounds, and two ballast outputs, plus the two relays themselves. That is it. Once you know those six points, the install becomes a simple matter of routing wires to the right places.
1. The OEM Input
The very first connection is the OEM input. Depending on what size you have, you might just have the harness by itself, or you might have an adapter included that connects to your factory bulb size (like an H11 or H1, for example). The OEM input goes to the output from your car that used to plug into the stock halogen light bulb. When you turn the headlights on, that connector sends a signal to the harness that turns on the relay, and the relays draw power from the battery and send it out to the ballasts.
You only want to do this on one side. There is only one of these on the entire harness, and generally speaking you connect it to the side that is closer to the car battery. This is because of the way all of the wires are spaced out on the harness, so putting the input near the battery gives you the cleanest wire routing.
2. The Fused Battery Positive Wire
Next, you have only one wire with the fuse in line. This goes to the positive battery terminal and delivers 12 volts. This is the power input for the relays, so that when the harness gets hit with the OEM input signal, it can draw power from the car battery, send it through the relays, and out to the ballasts. This single fused wire just goes to the one battery terminal on the car.
Because this one wire powers both relays, you do not need to run separate power to each side. The relays handle the distribution internally, which keeps the wiring minimal and clean.
3 and 4. The Two Ground Wires
Each harness has grounds, and in this particular version there are two grounds, one per side. This needs to go to a solid ground, and we cannot stress enough how important that is. We always recommend using factory grounds if you have them available. Generally speaking, a lot of things under the hood are grounded to certain points on each side of the engine bay, so those factory points are your best bet.
If you do not have a solid or factory ground available, you want to make sure you are grounding to bare metal, not plastic, not a rubber washer, and not to a painted surface. A poor ground is one of the most common causes of flickering or non-firing HIDs, so take the extra minute to find a clean, solid grounding point on each side.
5 and 6. The Two Ballast Outputs
The last connection on the harness is the ballast output, one for each ballast (driver side and passenger side). The one that is obviously a lot longer is going to go to the side opposite that input, meaning opposite the car battery. So that longer wire will run across the engine bay to reach the far ballast.
These are 9006 female outputs, which are very standard across the board on all of our harnesses. That is because all the inputs on our ballasts (and most ballasts on the market) are 9006 male. So one output goes to one ballast, and the other output goes to the other ballast. It really is that simple, and the standardized connector means you rarely have to worry about compatibility.
Mounting the Relays Correctly
Other than the connections above, you are just left with the relays, and there are just two of them, one per side. One does the driver side, and the other does the passenger side. There is nothing you really need to connect here, and importantly, the relays do not necessarily need to be grounded.
What you do need to know is this: the relay needs to be mounted so that when it is under the hood, the wires exit from the bottom. If you do that, no moisture will ever enter the relays and they should never fail. If you mount it sideways or upside down, water can come down the wires, seep past the rubber seals, and leak into the relays, which can cause a headlight to go out. So with the relays, just make sure you mount them vertically with the wires pointing down and you will be fine.
Putting It All Together
Once you have those six connections made (OEM input near the battery, fused positive to the battery terminal, both grounds to solid metal, and both 9006 ballast outputs to their respective ballasts), and the relays mounted vertically, the install is done. We like to do a quick test with the headlights on before tidying up the wiring, just to confirm both sides fire.
This particular harness is the low-beam version, but the HD relay platform covers a wide range of bulb sizes. If your vehicle uses a different connector, check out the HD Relay: 9012 Relay Harness or the HD Relay: H8 wiring harness, both built on the same reliable design. You can browse the full lineup of relay harnesses at TRS to find the exact fitment for your application.
For a few less common bulb sizes, our friends carry compatible options as well, including the Morimoto H10/9145 HD Relay Harness and the HD Pro Relay Harness for 9005 bulbs at Headlight Revolution. No matter which size you need, the wiring approach we covered here stays exactly the same: six connections and two vertically mounted relays.
Frequently Asked Questions
What connections are on the Morimoto HD relay harness?
The harness includes six connections total: one OEM input, one fused positive battery wire, two ground wires, and two 9006 female ballast outputs, plus two relays.
Where should the OEM input be connected?
The OEM input plugs into the factory connector that once powered the halogen bulb, and it should only be used on the side closer to the car battery.
How is power supplied to the relays?
A single fused wire connects directly to the positive battery terminal, delivering 12 volts to power both relays.
What type of ground connection does the harness need?
Each side has its own ground wire that should be connected to a solid factory ground point on bare metal, avoiding plastic, paint, or rubber surfaces.
How should the relays be mounted to prevent failure?
The relays should be mounted vertically with the wires exiting from the bottom so water cannot enter and cause the relay or headlight to fail.
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.
