I have a tent trailer and the lights don't work right. It has a 4 wire connector to my car but one of the wires on the trailer connector was worn out and probably not working. So I got a new 4 prong connector for the trailer but that 4 wire adapter goes to 6 wires on the trailer. This particular trailer has what I call "surround" lights in addition to turn signals and brake lights.

So, one wire is "common" and I've used alligator clips to hook up and test what the other 5 wires do. One of them does nothing and I think it's because one of the connections in the back are not connected and today I'm going to try and take off the back panel to get to the lights (the light fixture itself is riveted on).

So, what should each of the wires do and how should I hook them up to my 4 wire system? Also the brake and turn signals are the same bulb just different filaments. I call them the "small filament" and the "big filament" even though they are the same size. When the parking lights are on, is that the "small filament" on and then the brake and turn signal activate the "big filament"? And that can't be right because what if I'm stopped and have the turn signal on. I'm confused as you can tell.

1 Answer 1


Here is a diagram for the basic 4 wire trailer.

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  • Yellow - Left Stop/Turn (the big filament as you described in the left bulb)
  • Green - Right Stop/Turn (the big filament as you described in the right bulb)
  • Brown - Parking/Marker lights (the small filament as you described in both the right and left side bulbs)
  • White - Ground

They all hook to the respective bulbs on your vehicle, so when you are using the brakes and the turn signals at the same time they are mimicking your vehicle lights.

The other wire could be for the surge brakes if equipped. They would be hooked into the backup lights on your vehicle to inhibit the trailer brakes while backing. It requires a 5 wire connector that yours did not come equipped with.

Other options include:

  • Brakes, to apply electric trailer brakes
  • 12 volts, for accessories or to charge trailer batteries
  • backup lights, same as above but for actually turning on backup lights

I am not aware of any other standard wires/circuits that are used on trailers

You can use a fused jumper wire and apply power to each of the wires with the common lead hooked to ground and see which lights light up if they are not color coded.

Another diagram showing how to wire up to a 7 wire connector

enter image description here

  • Wow. Just Wow. I'll go over this later and let you know how it turns out. One of my biggest problems was that a bulb wasn't making good contact and that threw me off!
    – tooshel
    Mar 21, 2011 at 1:43
  • I just hate it when the obvious thing (like checking bulbs are tight) is not obvious when starting a troubleshooting job! Check wiring, fuses, connectors, hmmm. Check bulb! Jul 1, 2016 at 18:04

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