You're right, the rear door speakers sound very muddy. A number of us on this forum have replaced them with something better. The rear door speakers are pop-riveted in, you will need to drill out the 3 pop rivets that hold each speaker. The speaker hole in the door seems small but if you use speaker adaptors you can fit 6 1/2" diameter speakers. I made my own adaptors out of 3/4" thick particle board but you may want to use MDF instead, or you can buy a pair of adaptors.
I went with component speakers and put the separate tweaters up high on the rear doors, it makes a world of difference. The sound stage has so much more "presence" now. Have fun!
I went with component speakers and put the separate tweaters up high on the rear doors, it makes a world of difference. The sound stage has so much more "presence" now. Have fun!