What I would like to do is put in a 5 channel amp, upgrade the front and rear door speakers and add a sub. I would like to keep the head unit and the tweeters in the dash. Has anybody accomplished this? Any problems? I want to get some big sound, but Id like to keep the head unit and steering wheel control if possible. Is this possible?

 

Possible, but be careful what speakers you choose for the front, 3" is about the deepest you can go without building them out from the mount. It's not really the window that's the issue, it's the track.
LookHERE
This is my recent expericence with the infinity system. I swapped all 4 of the door speakers, but wanted to keep the crossover and tweeters.
I had started thinking I was going to do the same as you, once I put the mew speakers up front (after figuring out what was up) I decided the fronts were plenty loud enough for me, and I decided to just get an amp for my sub instead.

 

thanks for the help. I beat this one to death and decided Im going to get rid of the amp all together. Get rid of all the speakers and replace them with a couple of amps, sub, and some decent crossover speakers. I was trying to save the tweets for some crazy reason. I thought I could save the factory amp and let it drive the tweeters,but that is just plain crazy. What an expensive option for the infinity system. Ill take some pics if anyone is interested.