How to wire a trailer – if your car has separate signal and brake lights

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Here is my problem. I have a flat 4 wiring harness. My car is ‘modern’ and has separate signal and brake lights.

So, I can wire it to do the following:

  1. have brakes and signal but NO running lights
  2. OR have running lights and signal lights but NO brake lights

Now, I’ve been using it sans running lights since during the day its not necessary to have them, or legally required. So option #1 is really the only one available.

However, the correct fix is to get them all working.  So again, the problem is that the brakes and signals work off different lights on the back of my car (2007 Dodge Caliber).

The option is to install a diode on the brake / running lights wire. Whats a diode? do you remember from your highschool or college electronics course?? A diode is a thing that allows electricity to flow only one way.

Details on how to install the diode in a trailer scenario is listed here:

http://www.etrailer.com/faq-diode-installation-instructions-for-separate-tail-light-systems.aspx

You need to connect the signal and brake lights from your car together and to have them flow to the trailer but if you simply connect all the wires, when your signal comes on, your brake lights would light up and vice versa. Not to mention possible damage to the car electronics if they are doing anything funky there with the lights. So the diode prevents power travelling back up the line to the incorrect light (see image above).

You can get these I believe at radio shack (the source now :/ ). YOu can even scavenge them from old electronics if you know what to look for like old vcrs and such.

I’ve yet to do this, but will be getting some diodes soon so I can get this all fixed up. Most of the trailer season is over now so this will be a winter project.

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