The rear door speakers are cheap paper-cone 5.25"s.
Because you need a mounting plate, which keeps the speakers away from the door a little, you can put in 6.5"s.
It's pretty straight-forward to replace them. If I remember correctly, pop the D-shaped cover off the door handle to reveal a screw. There is another where you grab the door to close it. Then the whole inside just pops off. Pay attention to the cables for the lock and latch.
The speakers are held in with pop rivets. Just drill them out. Make or buy a mounting bracket and put in your new speakers.
Here is a link
to a diy for the front speakers. The rears are identical except the hole in the door is a little smaller.
A few extra things I did:
1) I made my speaker adapter plate out of mdf because it is easier to work with and I had some scraps laying around.
2) I also put several coats of paint on the adapter plates. Water will get in the door and you don't want it to ruin your hard work.
3) I also had some scraps of fun foam from a craft store. It's almost like a sheet of gasket material. I put a ring of that between the door and the mdf. (The speaker already had a gasket to go between it and the mdf.) Sound deadener would be better, but I'm too cheap. (I will probably do some eventually though.)