Since the left turn and brake on that side does not work, that points to the same wire not working. The brake is the same filament in the bulb as the left turn, just the turn blinks and the stop stays on constant.
Trailer Wiring Diagrams | etrailer.com
I'm making an assumption you tried a different light bulb, yes/no?
If you have a bulb that you know is good and still does not work on the left turn/brake then get a volt meter and check for 12 volts on that wire terminal with the left turn signal blinking on the truck.
Your color codes do not seem to match industry standard as yellow is suppose to be the left turn,left brake wire and it is not hooked up. Which may be the problem....
Left light should be yellow (brake and directional) and brown (marker lights) to the bulb and right side light should be brown (marker lights) and green (right turn and brake) to the bulb.
The red, that is not normal.
Do not know if the wires are mixed up or someone just used different colors.
Your going to have to ring out the wires starting at the 7 wire trailer plug and figure out which one ends up where on the brake lights. Get a long piece of wire, put in on the correct pin in the 7 wire plug for left turn then head to the back of the camper and see if there is continuity at the left turn/brake terminal that the bulb plugs into.
I'm assuming this camper is new to you and you are trying to get it to work for the 1st time since you acquired it? Is this correct? If not and it worked correct for a long time and now not, that points elsewhere.
Any chance of posting some pictures of the light fixtures wiring?
There are no circuit breakers in the turn, stop or body lights as standard unless you have a real old camper. What year/model is it?
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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