wiring harness

I have a 98 bravada and want to wire it to pull a camper with a 7 wire plug - electric brakes and aux power. the harness that lives behind the right taillight is currently wired for a 4 pin connection, but there are a bunch of other wires in the pigtail. does anyone know what the rest of the wires are for? I am hoping I can use them to send the aux power and brake voltage back to the trailer without having to run more wires back.

this site might be helpfull..
http://www.etrailer.com/faq/wiring.asp