Originally Posted by Bumpncarstereo
what is the extra 3 pins for? my truck came pre wired with a 4 pin.
Yeah, one is for Trailer Brakes, one for 12V Source to charge a trailer battery or run electronics from the truck battery, and the other is usually labeled "aux". I have my "aux" hooked to my reverse lights. On my boat trailer, it locks out the hydraulic brakes when driving in reverse, on my camper, it actuates a relay for back-up lights on the back of the trailer.