Diode kit for use when towing my Prius

I have a 2007 Prius that I occasionally tow behind an RV on a car dolly. I’d like to install a ‘diode kit’ so that I can use the Prius’ brake and tail lights as the "trailer lights" when it's being towed. Bottom line is that I need to find a place where an external 12V source can be applied that will light up (independently) the left tail, right tail, left brake, and right brake lights.

So, I need to know where the 12V connectors are for the tail light and brake light on each side. The tail lights are easy, they are incandescent, and there is a simple 2-wire connection for the bulbs. The brake light, however, is interesting. There are four “lights” in each assembly - from top to bottom - brake, turn signal, reverse, and tail. The bottom three are incandescent bulbs, but the top is an LED assembly. There is, at least from what I can see when I remove the little cover behind the light assembly inside the car, no connection for the brake LED lights at the top of the light assembly. There is, however, a 6-wire connector between the reverse and tail light bulbs, which I’m guessing is the connection for the brake lights. 5 out of the 6 slots in the 6-wire connector have wires.

So, why 5 wires? Is one of them ground and another the 12V supply that is on (or off) when the brake light should be on (or off)? What are the other 3 wires for? Can I apply 12V to one of them to make the brake light LEDs light up? Anything else I should be worried about?

Thanks!

 

I wanted to report back what I ended up doing in case this is helpful for someone else.

Goals:

1. End up with a flat-4 trailer connector at the front of the Prius that can be plugged in to a tow vehicle (RV)
2. Use the Prius’s tail and brake lights as a “combined” system when it’s being towed- i.e., the Prius brake lights are both brake & turn signals
3. Don’t cut any factory wiring
4. Don’t change any Prius behavior when the Prius is not being towed

NON-goals:

1. Use the Prius’s amber turn signal lights when being towed
2. Use the Prius’s third brake light when being towed

Side note, there is a kit that “separates” the combined signals from a 4-wire trailer connector into separate brake & turn signal lights. This kit could be used in addition to what I did, and would “fix” the NON-goals listed above. Why didn’t I do that? While this kit got mostly good reviews, there were a couple that scared me away - some said the lights were dim (but that could have been an issue with the towing vehicle), and others said that the turn signal flashing was not in sync with the tow vehicle’s flashing (which for some reason bothered me). I cannot confirm whether these issues are real since I didn’t do that. If you decide to go this route, you’ll need a 6-diode kit instead of the 4-diode kit I purchased.

The key to not cutting factory wires is the 6-wire connector I asked about in the original question above. It is actually the connector for all of the bulbs/LEDs in the tail light assembly. Most of the wires come right back out and connect to the bulbs themselves. That means I could install “jumper” connectors in between the factory connectors, and tie-in to the correct wires in the jumper without cutting any factory wires. Remove the jumper, and the factory plug will still plug right in to the tail light assembly directly, just like it does from the factory.

Parts I purchased:

1. Roadmaster RM-154 diode kit
2. Tekonsha 118308 wiring harness. I only used the two 6-pin wiring couplers and some wires, but threw away most of the kit. Note that this kit is for a Camry, but I bought it instead of the Prius-specific one because the parts I needed were identical between the two kits, and the Camry kit was cheaper for some reason.
3. Larger ring terminal, the one that came with the diode kit was too small for the ground bolt in the back of the Prius.
4. Wire loom. The diode kit came with 3’, but I wanted more

Steps to install:

1. Run the 4-wire flat 30’ cable from the front to the back. I decided to go along the driver’s side mostly inside the vehicle. I removed the fender cover that’s above the driver-front tire, and routed it through the firewall in the hole for the hood-release cable. I used wire loom from the firewall to the connector in front, so all of the wire in the exterior was loomed. I used cable-ties to hold it in place. Then I just routed the cable under the door sills to the back. There are lots of posts on how to do this. I really like that most of the wire is inside the vehicle, yet completely hidden.
2. Use the directions at event's web site, the prius_trailer_wiring.htm page to re-wire the 118308 wiring harness. Note that the short white wires should be for the ground, reverse, and turn bulbs. The colored wires are for brake and tail bulbs. This is a little different from that web site, as this is a different application. So use his instructions for how to remove the wires from the connectors, but don't reinstall the wires in the same locations he did. I cut the wires in this kit off near the black boxes and threw away most of that kit. I wired in the diodes to the wires that used to go to the kit’s black boxes. Note that 3 of the wires are just “pass-through” short wires, and the other 2 go through diodes, but are essentially pass-through too.
3. Connect the wires on the 4-wire cable to the appropriate diodes
4. Ground the 4-wire cable in the same location as evnut.

In the end, when I’m towing the Prius on the car dolly, there are three sets of lights that work in unison (RV, car dolly, and Prius). Why did I want both the Prius and dolly lights to work (i.e., aren’t the dolly lights enough)? The dolly lights are about even with the Prius’s front doors, and I wanted lights at the very back. Not sure if that was necessary from a legal perspective, but I thought it would be best.

The other thing I did was to replace the original 7-pin round trailer connector on the RV with a “Hopkins 48470 Endurance Multi-Tow Vehicle Side Connector”. This new connector has both a 7-pin round connector that the car dolly connects to, and a 4-pin flat that the Prius 4-pin connector connects to. This makes the trailer wiring connections easy. The dolly needs the round connector because it has trailer brakes in addition to lights. Note that the diode kit comes with a 6-foot 4-wire flat cable with appropriate connectors at both ends, which is used to connect the Prius to the RV.

Thanks, and let me know if you have any questions!