1). Contact Metra and find an authorized dealer in your area. Metra.com (I think).
2). Depending on if you have on-star or not you'll need to get a corresponding harness.
a. If no Onstar you'll need the Harness number GMRC-04 (check with Metra to make sure.
b. If you have Onstar I don't know what harness you will need ( they will though).
3). I ordered mine from a place called Lafayette Radio. A shop in Florida.
Then just get an install kit( I reccommend not skimping on this If you have to pay a little more for a better fitting kit do it. I got one from Wal-Mart the first time and the fit was awful.
The good thing about these harnesses is that it retains the stock features of RAP, &
the door chimes( although they are abit louder).
If you don't care about these features you could just hard wire it in. Several people in the forum have done that successfully.
Hope this helps,
02 Se with a few mods,
Coming Soon 1.9'* OR pushrods and other stuff