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Tow Lights and Wiring

There are several options for rear lights while towing. Light bars and separate lights are one method.
Using the towed vehicles existing lights is another method.

A light bar can be used if towing for a limited number of times. There is little labor and time involved in set up.
Since most RV’s have a wiring connector already in place, you can simply run an extension along the tow bar and back to the lights.
A magnetic mount is generally used to attach the lights to the vehicle. Storing a light bar, installing and removing a light bar become tiresome if towing frequently.
Wires dangling down and difficulty securing them is another good reason to hard wire the towed vehicle’s rear lights.
There are now wireless tow lights available. I have not tried a wireless light set up, so I will not comment on them at this time.

Most often the rear lights on the towed vehicle are hard wired to work off the RV. Wiring kits are available from most tow bar manufacturers.
These kits include wiring, connectors, and diodes. The diodes are necessary to avoid electrical feedback through the towed vehicles electrical system.
Due to the design of the automotive electrical system, a turn signal switch or a brake light can feedback through a vehicle possibly damaging the system or running an unwanted circuit.
A diode simply isolates the current flow to the direction of the light and not in reverse.
The diodes designed for towing should be used since they are designed for exterior use and typically have a heat sink built on the diode.

Hard wiring tow lights can be very complicated and difficult. In some applications, separate light bulbs may be needed.
Vehicles that use ground switching to activate lights will require separate bulbs.
Separate bulbs can create problems since there is usually not enough room for another socket and bulb.
Be sure to check your rear brake lights for voltage before deciding on which towed light system to use.
Tow vehicles use positive switching to activate the towed vehicle lights and cannot activate negative switching lights.
In other words, a vehicle that uses ground switching has positive power at the rear light when the light is off and uses ground continuity to activate the light.
You can test for ground switching with a simple voltage test at the rear lights. I have found a couple of vehicles that use ground switching for brake lights, but not running lights.
Remove the rear light fixture and test for constant voltage at the positive feed for the brake light. If it has constant voltage with the light off, it is ground switching.
If it gets voltage only when applying the brake, it uses positive switching. It is important that you understand you will be using the red brake lights.
I do not recommend using amber turn signals for towing. Some RV’s have rear amber lights.
They do not typically use an amber circuit for towing, but rather a converter for red light only use.
You can have an RV with amber turn signals and still use the red brake lights only for both brakes lights and turn signal lights on your towed vehicle.

I make my own wiring kits. I use bulk wiring and have a bulk supply of diodes. Roadmaster sells their diodes in bulk bags and they are very good quality.
Using a bulk roll of wire eliminates the problem of the wiring being too short. I find that Remco supplies a harness that is too short for almost all vehicles.
Most auto parts stores have 4 wire trailer wire in bulk. Generally I would buy about 30 feet to be sure you have enough. You don’t want to splice the harness.
I usually run my wiring outside the vehicle. It is easiest to run the harness back while the front fascia or bumper is off during base plate installation.
Find a good location for the light socket. One of the reasons I like Blue Ox base plates is because they provide a mounting point for the socket.
You can use any type trailer socket you’d like. I usually figure how many wires in total I will be running before choosing a socket.
There will be 4 wires for lights. If using a lube pump, there will be 2 more. If using a braking unit there may be more, and if using a battery charging system, there will be more.
I will cover charging systems and braking systems in another article.
Mount your socket, but do not connect any wires yet. Start to run your 4 wire harness to the rear. Lead the harness into the engine compartment.
I usually go to the driver’s side. Stop at an area at the front where you can attach the ground wire to a good ground source.
I look for a factory harness ground point. Split the white wire off from the 4 wire harness and cut to length. Install a terminal then attach at ground point.
Continue to run the remaining 3 wires to the rear going down near the firewall then to the under side of the vehicle.
There are usually brake lines and/or fuel lines to secure the harness to.  Be careful to avoid sharp or rotating objects and areas that will get hot. Support harness with tie straps.
When getting near the rear decide how the harness will reach the lights. Remove the rear light fixtures to see how the factory harness runs to them.
If there is outside access to the factory harness, run the 4 wire harness directly to that area.
If the factory harness is inside the vehicle, you will need to find an easy access point to run the harness through.
Typically I find trucks and sport utility vehicles have outside harnesses and cars have inside harnesses.
I look for a factory grommet or vent flap to run the wire to the inside for cars. Be sure to poke a small hole and seal it with silicone seal.
Split the green wire off from the 4 wire and run it to the right side rear light. Run the other 2 wires to the left side rear light.
Using only the red brake light (not amber), find the factories wire that feeds the left brake light. Be sure to test for voltage right next to the light bulb while someone applies the brake.
Some vehicles use “bulb out” indicators. You will need to be on the bulb side of the indicators.
Cutting the brake light wire very close to the bulb/socket will ensure you are on the correct side of a bulb out indicator.
Cut the wire; install the terminal connector provided with the diode to both ends of the cut wire.
Be sure to use dielectric terminal grease to all terminals and connections. Poor or corroded connections are the number 1 problem I see with tow lights.
Connect the wire coming from the vehicles harness (feeding the light) to a diode input using the diagram on the diode for proper electrical flow.
Run the yellow from the new harness to the other diode input. Install a terminal on the cut wire (bulb side) and connect to output diode terminal.
Follow the same procedure on the right rear red light using the green wire from the 4 wire harness.