GeeNellyGMC

I've got a 2003 Sierra SLE Extended cab. Can anyone tell me about upfitting connections in the under hood fusebox and others. Would my truck have a "7 pin harness" somewhere tucked up near the spare, or would that have been an option. I found the 4 pin harness. Under the hood and in the rear would there be power and brake actuator wires related to the trailer? I also wonder if installed the factory fog light switch would the bcm have to be programmed to enable this under the hood? Thanks