Trailer Light Wiring

I have a 2006 E350, V10 and just added a class IV hitch and was ready to wire for the trailer lights. I found a harness with a flat (4-pin) plug at a used auto yard. Under my van, I found the plug that was identical to the one in the yard (it was capped) and plugged it in. I am getting nothing through this plug. Is there some sort of fuse or relay that needs to be connected or added in order to get juice to the trailer lights? Thank you.

In the underhood fuse block there is a 20 amp fuse #26 (turn signal) and a 20 amp fuse #24 (park lamps) and three relays (left, right, park lamp). Make sure the relays and fuses are there.

Thanks. That was the problem.