Believe me, you really really do want to replace the stock deck. It's really a bed performer in a lot of ways. I couldn't believe the difference when I replaced the HU in my 2000 GMC truck without changing the speakers. I did later on, but I did it in steps. HU, then the front speakers, then the rears.
Changing the HU was by far the best bang for the buck. The HUs in GM vehickes around that period sounded very bad. There used to be a site where they did a test report on several OEM head units and the GM ones came in dead last. I think the one in my truck put out 1.5 watts per channel @10% distortion, it was that bad. It wasn't just distortion, the FM was weak, and it had problems with any CDs that had the slightest nick on them. Ones that refused to play at all in the stock unit played fine in any aftermarket HU I tried.
I put a Sony HU in that truck and it is in a friend's car now, and doing fine. My present truck had a JVC high end HU, but it had issues, so I replaced it with a mid range Panasonic. No, it doesn't sound quite as good, but it works fine and costs about 40% what the JRC did. I have a friend working at one of the local car stereo stores and he says that Panasonics just don't come back, they seem to have no problems, unlike some of the other brands that are much more expensive. I have 2 Panasonic cassette decks at home that are old timers that just keep going. The newer of the two is almost 28 years old, and the older one is 36!
