Do a search on the sound system.
I think PELKED1 ??? (can't remember the correct name) did an excellent write-up on adding capacitors to the mid-range door speakers. A lot of people (including myself) have done this mod and everybody loves it. Costs about $2. Then you crank the bass up to 6, 7, 8 (whatever your preference) - now you can feel & hear the rear speakers.