are you running all 4 speakers from a stock unit?

My friend has the basslink in his 94 LX, major concern is placement of the thing. It really didnt give that great of a low end punch... even with it's integrated 200watt amp.

And about your idea of using a smaller speaker.. you'll need an adapter to use a smaller speaker... I dont see where the space/gaps would come from the adapter is supposed to fill in the bigger hole make fit for a 5x7 or whatever...

Rear speakers are not just for fill IMO, they give music spatial ambiance. But the consensus around here and I agree is to have components up front, rears the same in the back.