I have a 2004 F150 Lariat w/factory trailer 7 prong plug-in and hitch (so I assume towing package).

I bought a new horse trailer and all lights work except the interior lighting. I believe the lights would get their power from the full time hot prong on the plug-in but I can get any power there. I checked the fuse (#106 "trailer tow battery charge" in the owners manual) and it is good, tried different fuse just in case, even tried to jump the prongs, nothing. Tried adjacent empty spaces in fuse box thinking something was miswired or mislabeled, nothing.

Is there a relay somewhere or something I'm not seeing? Like I said, everything else works and checks out.

Thanks
Figgy

 

there is no "full time hot" prong on the 7 prong harnesses.

you did have the key in the run position when you were doing your tests, right?

 

Figgy, 02SC4x4 is right. The "hot" tap in the trailer plug is only hot with the ignition key ON. If you put the positive lead of a voltmeter into the tap at 1:00 o'clock and the negative lead into the tap at 7:00 o'clock, you should see battery voltage with the ignition ON.

- Jack