How-to: Reverse lights as loading lights with the hatch open

I fly powered paragliders, and almost always end up packing up after sunset. For that reason, I was looking into adding lights for rear area illumination. I realized we already have a rear area illuminator- the reverse lamps! So with a diode, a relay, some wire, and a bunch of connectors, I now have reverse lights that turn on when I open the hatch. I have also done the double dome light mod, and I'm happy to report the reverse lights are only active with the hatch, not the front doors, while all dome lights are still synced up.

This is some moderate level electrical work, not for the squeemish. You'll be cutting factory wiring. Don't blame me if you burn the car down. I do this kind of work for a living.

Video demo:

Tools required: Stripper/crimpers, multimeter, optionally soldering iron
Parts required: 1 automotive 5-pin relay, diode ~2A, 6 ft of wiring, spade and butt connectors, taps, heatshrink, optionally solder

Step 1: Locate your wires. In my 08 EX MT, the reverse bulb wire is helpfully located directly behind the glovebox on a gray plug. It is a green/black wire front and center. Use your meter to verify you have the right wire, it should read 12v when in reverse and nothing or ground with the car otherwise running. Auto transmission? I'm not sure, you'll need to find a wire that feeds both tail lights. Should be somewhere near the driver or passenger fuse blocks. Also, we will be utilizing the rear done light trigger, which is the green/white wire often used in the dual dome light jumper mod. It is on the right side of the wall behind the glovebox.

Step 2: Cut the wires. Yes, we have to sever the factory wire. We will reconnect them with caveats soon. Try to leave yourself a few inches on both sides. I took the liberty of extending both the reverse and dome wires after cutting them, giving me 6+ inches of slack to work with. If you are doing, or retaining, the dual dome light jumper you need to attach it to the side of the wire still in the plug, the side I helpfully labelled as "to dome light".

Step 3: Install the diode between the dome light side and hatch trigger sides, with the diode's grey band toward the hatch trigger side of the wire. Also install a wire to trigger the relay from the hatch trigger side of the diode, seen in red here. Be sure to heat shrink the diode to keep it from shorting against dash components.

Step 4: Grab a +12v constant source, one that stays on without a key in the ignition. A sure bet is a white wire under the steering wheel that goes to the ignition cylinder and fuse block. It's pretty thick. I personally used the 30 amp constant that feeds my factory amplifier, which is conveniently placed just below the glove box in the passenger kick panel. Whatever you pick, make sure it can supply 7-10 amps to run the reverse lights. Run this wire to the glove box location. I chose a yellow wire for clarity and ran it from the +12v amplifier wire (which was blue/orange).

Step 5: Relay connections. I have numbered and labeled them in the following picture. Best to show you than explain it.

Step 6: Pretty it up! The glove box moves in this space, make sure you have all loose wires ziptied up and out of the way. If you want to disable or enable this feature, you can add a switch between the constant +12v source and pin 87 of the relay (yellow wire in my example), or between the rear hatch trigger and pin 85 (red wire in my example.) Perhaps mount this switch on the back of the glovebox? Or get adventurous and snake some wires to the rear cargo area to place the switch there. The world is your oyster!

So what's happening here? Well with no ground trigger to trip the relay, pin 30 is connected to pin 87a which is basically just reattaching the revere wire we cut in step 2. Reverse lights function normally. But when we open the rear hatch, pin 85 is grounded. Since pin 86 is connected to +12v, this triggers the relay. Pin 30 (the reverse lights) are now connected to pin 87 (+12v) instead of the original reverse relay feed. We do this to eliminate the possibility of damaging the relay or and computerized components upstream from where we cut the reverse light wire. And what about that diode? Well it's there to only allow a ground signal from the rear dome light into the front dome light, but NOT allow a ground signal to flow from the front dome light to the rear light. If we didn't use that diode, every time you open a side door it would also turn on the reverse lamp. It's there to make sure the reverse lamp only powers up with a rear trigger is seen as opposed to any trigger.

So there you go, if you find yourself wanting a little more light behind your E this no-accessory-added wire mod might be for you.

Video demo: