questions for Carmelfrapinator

[vtdobes]

I was on the Circuit City website looking for the parts you mentioned as I have a 03 Silverado that I want to install my Alpine CD player in. There are two different parts and I was wondering if you knew specifically which one I need.
There is the Metra GM Class II Vehicle Wiring Harness (MET VTGMRC01) and the Metra GM Class II Data Interface (MET GMRC01). Both are the same price, but as I'm not very informed on this sort of thing I was confused on the difference between the two.
Also, in the post from "shoeish" he mentions the metra kit 99-3301. Is this the Metra 2000-Up Chevrolet Cavalier/Impala/Monte Carlo Installation Kit (MET 993301)? And if so will it work with my Alpine deck? You said you used the American International GM-K420 from Best Buy for the install kit for your JVC deck. Does it matter either way?
Do you think you can help me with these questions? I hope I'm not being to much of a bother, but as I said I don't know much about this and have never done it. I don't much trust the salesmen at these places to fully answer my questions. Also the store I bought my CD player from two years ago said it would cost me $150 for the parts to install in the 03 Silverado plus $35 for them to do it. Yeah right!!
Thanks, Cat