if youve already swapped out your head unit, you can use the rear speaker outputs to power the bass shakers. i had the bass shaker "pro" in my previous car; they did their job pretty well. it was a bit of a pain to unupholster the seat to installthem, but maybe you can find a better place to mount in the crossfire. In my Miata, installed INSIDE the seat was really my only option.

www.partsexpress.com has them for cheap