Geo Tracker, Speaker Possibilities

Hi guys

I have a 1997 Tracker 4 door 4x4. There is a factory sound system in it now which sucks. I am wondering about the speakers. I am going to replace the double din factory tape deck with a nice head unit and get a bezel to make it fit etc. What I am concerned about is the speakers. Factory is 4 4x4 speakers. 2 on the top of the dash and 2 behind the rear seats. The back speakers are in plastic wall panels and the ones in the dash are in a hard to get to spot because of the windshield angle. I am not too worried about the front speakers. I can just put a better set of 4x4's in there but I am wondering what I can do with the rears. Are there any kids or adaptors I can buy that will make 6x9s fit into the factory holes? By this I mean the magnet for the 6x9 would go into the hole for the factory speaker and the cone etc would protrude out of the wall panel. anyone have any ideas on where to mount more speakers? the doors are awful thin.... Anyone have an experience putting sound systems in trackers? Any *practical* & *constructive* advice would be great.

Thanks guys. Love the forum.