If you get the gmos-04 then you'll retain whatever onstar capability you have left. The aftermarket radio will be a step up on the stock deck in all aspects and it won't cause any problems with the hud, etc. You'd still get warning lights but i would think that you want to retain as much of the stock features as possible. If you're gonna do it then do it right the first time that way you don't waste money re-installing it. Especially if you're paying someone else to do it.