[usmc1723]

I'm pretty dumb when it comes to electrical, so I could really use some help! I just got a 5th wheel to tow with my 1999 F350 diesel, but when I plug the trailer cable into either one of the two 7-pin connectors on my truck, none of the brake lights/turn signals/running lights work (however the brake controller/trailer brakes do work). I got a 7-pin tester and when I plug it in, I get no 12v power light and no left turn signal light, but all everything else lights up fine (and yes, I have the engine on when testing). I also checked the fuses under the hood and fuses #1 and #3 both show no voltage with my test light, even after putting in new fuses (however all the others do).

I haven't seen any posts on this forum that match up with this specific scenario, so does anyone have any idea what might be causing this, or how I should go about troubleshooting this?

Any help would be greatly appreciated!!!

 

[usmc1723]

So since I have two 7-pin connectors on the truck (one in bed/one on rear bumper) I just tried testing the 7-pin connector in the bed of the truck while the trailer cable was hooked into the other 7-pin connector on the rear bumper (tried with the engine running), and when I did it this way, not only did the 7-pin tester show that everything was working (including 12v and left turn signal), but also fuses #1 & #3 showed power with my test light.

So does this mean it's likely an issue with the trailer wiring, rather than the truck?