When we started wiring HID projectors into motorcycles, one thing became clear fast: a car relay harness just does not want to fit right on a bike. That is exactly why Morimoto built a dedicated motorcycle line, and in this guide we walk through the HID relay harness for motorcycles from the ground up. We tested it, we installed it, and we want to show you why the layout matters and how each wire ends up in the right place. Whether you are running a simple low beam setup or a full dual bi-xenon build, this harness gives you stable power and lets you keep using the factory controls on the handlebar.
Why a Motorcycle-Specific Relay Harness Matters
The motorcycle line of relay harnesses from Morimoto stands out from their other harnesses because, unlike car versions, these are made just for a motorcycle. The heart of the design is a compact central relay with the output stemming from one side and the input stemming from the other. That spacing is the whole point. On a bike you do not have the open engine bay real estate you get in a car, so the relay and its wire runs are ideally spaced for a bike setup. We appreciated how the compact body tucks away without fighting the frame or the fairings.
If you have shopped for wiring before, you know most harnesses assume a four-wheeled vehicle. That is where this design breaks away. You can browse the full relay harness category at The Retrofit Source to see how the motorcycle options differ from the automotive ones, and if you want to compare against the broader catalog you can also look through Morimoto’s full wire harness lineup. The MotoCycle harness is purpose-built, and that focus shows in every connector.
Understanding the Input Side
Coming from the relay, we have the input. Now depending on what model harness you have, this input may look a little bit different. It may be a three-pin connector like an H4 style, or it may be a two-pin connector like the ones that come on the H7 or H11 versions. This is the connector that receives the factory signals from the wire harness that currently powers the halogen bulb on the bike. In practical terms, you get the high beam and low beam signal along with a ground, and together those control the relay.
This is the detail that keeps everything working like it should. Because the harness takes its trigger from the factory bulb connector, your handlebar switches still do the job. Flip to high beam and the harness responds. There is no bypassing your controls, no aftermarket switch to fumble with. If your bike uses a two-pin arrangement, the MotoCycle H7 Dual-Output harness is a good example of how the input connector adapts to different bulb families while keeping the same relay logic.
Power and Ground: Routing to the Battery
Also coming from the input side of the harness is one power wire, which includes the inline fuse. This goes to the positive terminal on the bike’s battery under the seat. Alongside it is the ground for the harness, which goes on the negative terminal on the bike’s battery, also under the seat. Having both connections land at the battery is what gives your HID system clean, stable power instead of pulling everything through the thin factory headlight circuit.
One thing we noticed and really liked during our install: from the input wires all the way back to the battery connections, you have plenty of length. That means you can route everything to the back of the bike, run it through the frame, and land it under the seat without stretching or splicing. The inline fuse on the power wire is a small but important safety piece, and it sits right where you can inspect it if you ever need to. If you want a deeper dive into how a relay harness draws switched power while triggering from the factory signal, our step-by-step HID relay harness install guide breaks the concept down with automotive wiring examples that carry over to bikes.
The Output Side and What It Can Power
On the opposite side of the relay is the output, and this is what goes toward the headlight. Again, depending on what harness you have, you may or may not have this exact prescription for outputs. The version we worked with here is the most complex motorcycle harness, so it has four outputs: two for the ballasts and two for the high beams. That configuration is what makes it capable of running two ballasts and two bi-xenon projectors at once.
Telling the outputs apart is easy once you know the code. The ballast outputs will always have the yellow rubber grommet along with the black and blue wire. The high beam outputs use the black and red wire instead. That simple color and grommet system takes the guesswork out of connecting things at the headlight, which matters when you are working in tight quarters under a fairing.
Simpler Low Beam Versions
Not every rider needs the four-output monster. If you have a low beam harness like an H11 or an H7 version, you will not have those extra ballast and high beam outputs, but you will have basically everything else. The relay, the input connector, the power wire with the inline fuse, and the ground wire all carry over. So even the simpler harnesses give you the same clean, stable power delivery, just scaled to a single-projector or low-beam-only application. If you are running a bulb size on the higher end, something like the HD Relay H9 harness shows how the same relay concept scales for different loads.
Installing It Clean on the Bike
Once you set this all up on the bike, you have plenty of length to do everything you need. Our approach is straightforward: run the wires, zip tie the harness to the frame, and hide it under the fairings. Because you have that extra length, you are never forced into an awkward run. We routed the power and ground back to the battery under the seat, tucked the relay into an open pocket in the frame, and brought the output side up to the headlight.
A little wire mesh helps the harness blend right in with all the factory wiring, so the finished job ends up looking really clean. The end result is a system that provides stable power to your new HID setup while letting you keep using the factory controls on the handlebar, so everything works just like it should. That is the whole goal: a professional, hidden install that behaves like it came from the factory. For those cross-shopping relay options across different bulb bases, the HD Pro Relay Harness for H3 bulbs is another reference point for how relay wiring adapts to specific bulb types.
Our Takeaways After Installing It
After running this harness on a bike, a few things stand out. First, the motorcycle-specific spacing is not marketing fluff, it genuinely makes the install easier because the relay and wire lengths are sized for how a bike is laid out. Second, the input flexibility (three-pin H4 or two-pin H7/H11) means the harness meets your existing halogen connector instead of forcing you to modify it. Third, the color-coded outputs with the yellow grommets remove the confusion when you are connecting ballasts versus high beams under a fairing.
If your goal is stable, factory-like HID performance on a motorcycle, this harness delivers exactly what a relay harness should: dedicated power from the battery, a trigger from the factory signal, and clean control at the headlight. Whether you go with the full four-output version for two ballasts and two bi-xenon projectors or a simpler low beam version, the core benefits stay the same.
Frequently Asked Questions
What makes the Morimoto MotoCycle harness different from a car HID harness?
It uses a compact central relay with input on one side and output on the other, spaced specifically to fit a motorcycle layout rather than a car.
What does the input side of the harness connect to?
It receives the factory high beam and low beam signal plus a ground from the bike’s existing halogen wiring, which controls the relay.
What plug styles are available on the input side?
Depending on the harness, the input can be a three-pin H4 style connector or a two-pin style used for H7 or H11 bulbs.
Where do the power and ground wires from the harness connect?
The power wire, which includes an inline fuse, connects to the positive terminal on the bike’s battery under the seat, and the ground wire connects to the negative terminal.
What can the most complex version of this harness power?
The most complex harness has four outputs, two for ballasts (marked with yellow grommets and black/blue wires) and two for high beams, allowing it to run two ballasts and two bi-xenon projectors.
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.
