I have been tracing down the reason that I have no interior trailer lights and think I have found the problem ... I am missing a relay on my 04 f-150 for the trailer connections constant power for the battery backup that are in most trailers today and figure that is the problem... But since its late I didn't make to down to the local parts store I thought I would post a line here. Is this a dealer item or can I buy a relay at a discount parts store? There is no original to base a part number from...

 

It's a standard relay. Look at the socket & then check your other relays for one that fits there. You can collect them in junkyards from almost any make/model/year in the '00-up era.

 

Check the Towing and Hauling forum. The Fuse & Relay locations is a sticky at the top :

https://www.f150online.com/forums/sh...d.php?t=184757

Part way down is the 04+ trailer tow adapter install pages, and the Full size ISO Relay that Steve mention, the Ford part number is in there.
Autozone can cross this to a brand they carry fro about 12.00. The Ford one is about 18.00, just to get the name Ford on it.

 

Thanks for all the info... For something I will use only occasionally I can settle for the aftermarket item.