I am in the same situation with my '90 Corolla. I just put in a bitchin' Pioneer DEH-P6600, and almost immediately blew a couple of my factory speakers. I plan to put 6"x9" three-ways in the rear deck, and put either 5 1/4" or 6 1/2" three-ways in the front doors. While it's true that the speakers would now be pointing at your ankles instead of your knees if installed in the doors, the volume of sound produced by the larger and more powerful speakers and the fact that they are enclosed better more than compensates for their position. Don't bother with the 4"s in the dash...they will sound crappy no matter what you do. I wouldn't mess around with trying to splice a factory '00 Camry deck in either. If you want decent sound, you HAVE to go aftermarket. The fact that you took that deck OUT of the Camry illustrates this. I would not be surprised if this setup sounded worse than what is in there now.
The only problem with putting speakers in the front doors is clearing the window glass. The '88-'92 Corollas have pretty thin doors. If the speakers are installed too high in the doors and/or the speakers themselves are too deep, then the window glass will hit them when you roll the windows down. The best advice I can give is to try and find speakers that are as thin as possible, and mount them as low and as far forward in the door as you can. This should prevent any clearance problems, as near as I can figure. And rewire the speakers with good wire, don't use the crappy factory harness. The factory decks usually don't put out more than about 8 watts of power, and that is about all the factory speaker wiring can handle. On the other hand, most aftermarket decks peak out at over 50 watts. I''ll let you know how mine turns out!


Regards, Aaron