adding a prodigy brake controller

Just got a P2 Prodigy brake controller and do not have a clue as to how to install on my 03 avalanche with the factory tow package. Is there an existing harness? I installed one on an 04 F150 and it was simple as there was a wiring harness under the dash. Pretty much plug and play.

http://chevyavalanchefanclub.com/cafcna/index.php/topic,80184.0.html