Ahhh, the memories. My very first car was the same one. Gotta love those AE89s.
Anyways, on to your inquiry.
The rear speaker "grills" are actually held on pretty much the same way that your front bezel is. There are clips holding the speakers to the rear dash. Best thing to do is to take a flat-head screwdriver, wrap it with tape so as to not damage the "grill" and slide it in between the dash and the "grill" assembly. Gently apply pressure upwards. It will feel like you're breaking the d*mn thing, but that's natural. Just keep applying gently upward pressure and it should snap right off. If you feel so inclined, do this from both sides. it should feel a little better if you're concerned about breaking them.
Now, on to the speakers:
Because of the (extreme) slope of the rear window, getting the rear speakers out without being a contortionist is pretty much near impossible. I suggest that before you attempt this endeavor to go to a hardware store and see if they have a "screw driver right angle". I'm not kidding you. That's what you have to ask them. Here's a pic:
Anyways, this little tool will save you massive amounts of needless bending, swearing, and possible damage to tinted windows, if so equipped.
There will be four screws holding the rear speakers on to a big black plastic base. Unscrew the screws on the speaker and disconnect the connectors from the back. As with the "grill," pull upwards on the speakers to get them off. You'll see in the end that there are clips holding the speakers on in pretty much the same fashion as the "grill."
The largest size speakers that you can get for the rear dash, without significant modification, are 4". It's not much to work with, but when you combine it with a sub and an amp, you can get it to sound relatively decent. Are you also looking to upgrade the fronts as well?
Also, when you put in your new speakers, don't expect great sound. In all honesty, the sound dynamics and audio physics of the car don't really give it much greatness. To get relatively good sound, you're going to need a decent sub/amp combo and high-pass filters for the small 4" speakers. Most 4" speakers that you're going to get are going to have closed mounts, meaning that the space where the screws go in are holes. 95% of the time, that won't work. You're going to have to turn those closed mounts into open ones but cutting away the ends of the holes/tabs to allow
all the screws to fully lock the speaker into place. If you need pics, let me know. I'm on my 3rd 'Rolla and I've pretty much taken these cars apart on the interior.
Good luck!