3.5" in the dash & 4x10" in the rear deck, you can easily install 6x9"s in the rear using an adapter plate. On the dash speakers you need to make sure the tweeter doesn't stick up past the speaker frame, or you'll have a problem getting the dash cover back on flush. Speaker brands I like are Pioneer, Cerwin Vega, JL & Audiobahn.
As for decks, the stock opening houses DIN & a half sized head units which kinda limits your options if you want to use an aftermarket stereo of the same size. Pioneer makes a couple of DIN & a half models, as well as JVC if I remember correctly. You can also install a later model GM stereo (the Monsoon system from f-bods is a pretty popular switch) try doing a search in this forum for Monsoon for more info.
If you don't mind going with a "standard" or single DIN sized radio your options are nearly limitless & you just need to use an installation kit which is readily available online or at local stereo shops. Brands I prefer are Pioneer, Panasonic & Clarion.
Doug C.
