Am I Supposed to See 12V at the Tail Lights?

Last Train wrote:

Artum Snowbird wrote:
OK.. to be clearer perhaps... and this might make more sense. If you are looking at a double filament bulb, one is for the brake/turn and the other is for the running light. There will be two feeds to the bulb, hot for the brake, and a hot for the run. There is no negative in the fixture, only a ground. So, my estimate of what you are measuring might be the voltage drop across the two filaments.

Have you taken the lens cover off yet to see what is going on with the actual filaments?

Yes, absolutely. All of this has been done with the lens cover removed and the positive and negative wires exposed as they emerge from inside the back wall of the trailer. I am looking at the source of the power from the wiring harness itself.

Coachmen has provided a positive black wire and a negative green wire (green is a non-standard color in this application, isn't it?). And there is a very short white wire - which I mistakenly ignored in the past - which appears to attach to the base of the bulb socket. Should this be the ground?

I've got to run to an appointment in a few minutes, but when I get back on task I will post a picture that will surely be worth the proverbial "1000 words."

Thanks again.

Trailer wiring

I hope this diagram helps. You can see that on one side green is the positive used for brake/turn and on the other side yellow is used for brake/turn. White is the negative return/ground. Most trailers now take the white wire to each bulb fixture instead of relying on just the frame of the trailer to provide the ground/negative return.

Your running lights are all fed with a brown wire.