SE won't have the Bose stereo, he could have OnStar though. Either way if he does/doesn't it doesn't matter as the OnStar in those cars our obsolete and don't work anyway.
The only "special" connector he would need is the one to retain his chimes as they go through the speakers. I personally think those are stupid and not worth the $50 they charge, but again I don't need chimes to tell me when I'm screwing up.
Anyway, the real wire you have to worry about tapping is the "switched" amp wire. I know this sounds funny because, "Mouse you just said I don't have Bose." Correct, I did say that. However, the rear 6x9 speakers are still set up as "woofers" even in the base models and they are powered by an "amp".
2004 Chevy Silverado Z71 - 15% tint, FTDRL, All-on-high, 5000k lows/fogs, tinted tails, color matched grille, Sound by Kenwood.