I have a 2003 1500 Silverado. It appears to have some sort of factory installed wire harness for trailers that comes out behind the bumper. The problem is that I cannot find a mating adapter to bring this connector down to a standard flat 4 pin. I attempted to get both a 7 pin and 7 blade adapter that goes down to 4 pin but they are not correct. In the attached picture the connector on the left hand side is what is currently on the truck. How do I go from that to a 4 pin? Thank you!!!

 

It may be an aftermarket oddball, and if that's the case you'd need to find who made it, and get an adaptor from them.

You probably could get out easier if you got a wrecking yard OEM or just bought a new one from a Dealer.