there are several adapters (harnesses, modules) that you can go with, different brands, like peripheral, pac and some others. The one I, and a couple of other people on here, have used is the peripheral GMAH24B, heres teh link,
http://www.cardomain.com/item/PERGMAH24B , and if you would like to keep the door, key, etc chimes, you will also need the GMAHCM heres the link
http://www.cardomain.com/item/PERGMAHCM .
I have not had any major problems with this module, except for one thing, the is an alternator "whine" present when you use onstar. I dont think this is because of the adapter but it might be related to it, i think its most likely due to the ground used. The reson i dont think its the modules is because i initially did not have the whine, but then added some more electronics to the truck adn maybe overused a ground and now i have it. But it might be related to the modules because some others who have used the modules also have had this problem.
which unit are you planning on installing?