I've got a '98 Ram with the factory receiver hitch and 7 pole tow harness. For the most part, the basic 4-pole functions work (ground, turn, brake, tail) with exception of the tail lights.

I don't have a copy of the service manual (can't download it on dialup internet either), and I don't have a Hayne's or Chilton's (their wiring diagrams aren't usually very good anyway).

Anyway, the tail light wire coming out of the factory 7 pole connector only reads 0.02 volts when the tail lights are on. I removed all flex tubing and electrical tape up to the frame rail on the driver's side and found that the brown wire running from the back side of the trailer plug in goes into a relay that sits on top of the very back of the driver frame rail. This relay has 4 wires going to it, so I'm assumed the two smaller wires are the "switch" wires meaning they turn on/off the connection for the other two wires. Of the two smaller wires, one was red, and one was black with a yellow tracer. The one with black wire with yellow tracer is showing 12V and runs from the same wire that goes to the license plate lights. It's the other 2 wires on the relay I'm not sure about. One wire is black, one wire is brown. Again, the brown wire goes straight from the relay and down to the 7 pole trailer connector. I don't remember where the black wire went. Oh wait, yes I do--it went up to the driver's side tail/brake light bulb socket.

So, that must mean the black wire is the 12V postive coming into the relay, correct? I get nothing at it. I checked the fuses and they looked fine. There only seems to be a generic "trailer" fuse under the hood, and a basic "tail" fuse in the dash (the tail lights on the truck work fine). I would say that the relay is bad, but I can hear it click, and I would assume power is supposed to be going into it in the first place?

 

Bueller, Bueller, anyone, Ferris Bueller?

 

Well I would get a 12v test light and have a partner and test to see which wires have power based on blinker left/right, parker lamp, brake. This is what I had to do when my trailer harness wasn't working. Turned out to be a broken connection.

You may have a broken connection or bad ground going to the trailer setup.

 

I used a multi-meter, that's how I know I'm only getting 0.02V to the brown wire going into the backside of the 7 pole trailer harness plug in, and that I'm not getting any voltage going into the relay (except for the 12V that is there to turn the relay on). I'm trying to verify the 4 wire colors of that relay, and find out which one (presumably the heavier black wire) going into the relay is supposed to be the 12V power for the tail lights, and with any luck, where that power wire starts so I know how far back to trace it.

The only thing I can tell is that the black wire comes off the driver side tail light right at the bulb for stop/turn/tail (dual filiment bulb) but I don't know if it's actually supposed to be 12V or not. Normally, I would expect a black wire to be ground but vehicle harnesses can be any color these days.