I think the standard Mk4 speakers are 4" which is why they sound so poor.

The best way to get audio (anything reasonable) into the car, is to make custom panels either for the door cards, or for the kick panels. Audioscape used to build ready to go door panels for the escorts and they were brilliant.

Getting some custom built may be expensive but worth it (depending on who builds them)

I wouldnt bother putting anything in the rear personally, just concentrate on the front.

I would recommend putting some sound deadening into the car though, especially if you are stripping it all out!