Hi Goin Camping and NortrnNMguy;
I found that my 1966
Overlander International had a black wire from the tow connecter all the way to the back compartment. It is too small of a wire gauge to be used for the battery or charging, as it is only about 16 awg. I hooked this up to the BU lights pin on the connector and installed a single tractor light from Walmart. I also installed a similar light with a built in switch on the inside of the rear compartment door. These make late arrivals soooo much easier!
A note to NorthrnNMguy: I have a 99 F150 with the full factory towing package. My backup lights and all else work fine. Under the hood, there are three power distribution boxes beside each other. The two smaller ones have tape around them. The big one the owners manual tells you about; and two others. One of these is your relays for flashers etc. The other one, which is not very big, has fuses that go to your tow connector. I recently had a utility trailer that blew one of these (secret) fuses. My truck lights worked fine, but I had no turn signal power to the trailer. Hope this helps a little!