Amber side markers are not operating.

Thank you all for your answers, yes, the fused truck supplies power for running lights if the reds and tail light is operating, yes, then the connection at the main cable is fine. If the bulbs are parallel, I think they are, then the answer is a wire has come off somewhere that powers the ambers. This is an Attitutude Toy Hauler w/o a wiring diagram. I contacted the Company, in part: power travels first to Driver side front upper then L side marker, over to front R upper then R side marker. The power loss has to be between the front driver side upper and the junction box.I don't know gauge or color. I tried to remove a marker but they are packed solidly behind with a mass of putty? Which I don't want to disturb. No time now to dig further off to MT

Don't forget to check the ground. If the OP rig is fiberglas sides then there's a ground wire from the light to the frame, if corrugated sides then the light is probably grounded to the siding.

Just for giggles check your 7 way plug. Sometimes there could be a contact that is bent/ missing or some minor corrosion. Seems to be my problem most of the time.

The lights are in parallel and one bad bulb will not take out the entire string like cheap Christmas lights. The trailer DOT lighting is powered and fused by the tow vehicle. The junction box is on or just rear of the tongue of a bumper pull and close to the pin on a fifth wheel.

Thank you Bedlam for your post, will follow the harness back and find out why the ambers aren't operating. Question, don't know the term , but if one bulb is out, would it affect the entire light system, reds, tail lights or would it just be the individual bulb. Don't know where the splice box is, but the fuse panel below frig is a jumble of wires....does the slice box go through the fuse wall area? Or are the driving lights fused off truck?

Follow your trailer plug harness into the toy hauler to find the splice box. You may get lucky and find a broken wire or loose connection there.