Hey, been on this forum for a while as I have a '99 F250 but bought a 2019 350 transit and I'm trying to get trailer lights on the thing.
I did some reading around and found that there is a harness that runs to the back for lights, trailer brakes, ect, got the harness, connected it, tested the lights and nothing.
I thought, "well the trailer parking lamp relay is missing, maybe that's it". So I got a relay, tried to install it but there are no contacts to grab the relay so I guess no wiring in the back of the fuse panel even though there are fuses in place for the trailer wiring.
Anyone have any idea why there's no power to the trailer light wiring harness?
Speculation appreciated :P

 

Did you ever figure out your trailer wiring situation? I was in the exact same weeds on my '17 T350 a few days ago. I too tried to tie into that harness that terminates on the port side frame rail just back of the rear wheels, and got no life signs...scooched along on my back tracing the thing, and whaddayaknow, that harness terminates in another connector behind the front wheels. So evidently the factory build includes this center harness for rigging the trailer wiring, but not the front one that would go to the various relays and fuses. I tried to buy this front harness at the stealership, and they couldn't even find a part number for it.

So, went the old fashioned route, and scabbed into the left and rear turn signal wires and one running light wire in the tail lights, ran two full-time 12v positives fused off the battery all the way back, and grounded locally to the frame. I'm close...I get running lights, flashers and brake lights...but no discrete turn signals. Also I have a small mystery of both the truck and trailer's running and lights having a constant low glow when truck and trailer are connected. Each turn signal wire works to spec when attached individually on the meter...so something is either shorted or ungrounded. I did try a hard-wire test ground from truck to trailer, without effect. If there's anybody out there with intimate knowledge of the subject, I'll draw out how I have things wired and maybe my error(s) will be obvious. Dang, there's only 7 wires (well, 6 actually as I'm not using the blue "electric brake" wire)...how hard can it be??