I just changed mine on my 97 SSEi. I did not even consider even removing the seat, and I didnt need to.
I just pulled off the factory speaker covers. They tend to catch on the carpet but theres no nice way to get them off ! Be careful you dont pull too hard and pull the wires off the tweeter.
There is a quick connect plug that you can disconnect the tweeker from the cars harness. Once that is unplugged, you can totally remove the cover and you can see your 6x9'* there. They are mounted on some type of rubber mounting assembly. You can either remove that or just unscrew the 4 screws and remove the speakers. Its a bit cramped coz of the back window, but its pretty easy.
My Bose speakers had died and I bought some aftermarket speakers and they sound great. Most 6x9'* will screw straight onto the rubber mounting assembly. They sound great even with the factory Bose amp driving them. :P
I was fortunate that after I put the new speakers in, I put the original covers back on, plugged in the integrated tweeters and now I have a 4 way on each side. Its good too because the factory covers will tend to muffle the tweeters that are on the aftermarket speakers.
You will probably have to recalibrate your compass. Mine was stuck pointing to southwest until I did the re-calibrate procedure.
Hope that helps