I have a 98. 5 with the infinity system. RF speaker has been blown since around 24k miles. I thought it was a rattle in the door, after warrantee expired I relized if I turned off the radio, the rattle want away!
Anyway, the front will take most 6"X9" speakers, and the rear is something around 5 1/2" round. Check with crutchfield as to what fits. they may have the install guide on the web. In there books they don't list the infinity system in there replacment guide because of the factory amps.
Problem is the factory amps are mounted on the front speakers. I have a brand new set of speakers I removed from my 96 when I sold it. Next time I have an extra day I will try to mount the amps onto my aftermarket speakers and install them. Only problem is I been waiting for about 3 years for an extra day!
If its a rear speaker you are replacing, don't expect great sound from the back. I think the front amps also have a frequency cutoff network that only sends mid range to the rear speakers.
If its the front, replacing with aftermarket would be the cheepest route. Buy a pair of something like Jenson 6"X9" triaxles, and figure out how to mount the factory amps onto them. It would only take a handmade bracket mounted to the speakers.
If I recall, there was some heat sink compound on the back of the amps so the speaker body would help cool the amps. Not sure, its been almost 3 years since I looked at them.
NOTE: Buy some extra door panal mounting clips before you start. You will break a couple. and be carefull with the wire mounts onto the electric door switches, I broke mine last time I was in there.
Also while your in the door, I here the auto down window switch from the new trucks is an exact replacment for the older switches, if your into that kind of thing. There was a thread about 9 months ago with part numbers of what fit