a past owner took off the 7-way at the back bumper installed new one but cant seem to find constant 12v wire to run asseries on trailer:

 

The +12 in the 7 pin trailer plug are only hot in the run position.

If in the run position, the pin is not hot, check the fuse and relay for the trailer tow battery circuit. The previous body style truck had the user install quite a bit of the trailer tow electrical items, and the P.O might not have put them in ??

Sticky at the top of the towing and hauling forum with the correct relay and fuse locations.