Ian - Did you ever look into something called "bass shakers"? I have seen
several versions, Crutchfield has some that are about 5" in diameter and 2"
high. You screw them to the floor and supposedly they turn the body of your
vehicle into a large speaker. They aren't self-powered, you need to supply
your own amp. They run about $180 or the "pro" model is $230. I haven't heard
them action, I am hoping someone else will be a guinea pig.

Derek
'87 Syncro