3 12's will fit in the back of a nissan no problem as long as they are sealed boxes. If you remove the back seat and glass the bottom of the car you can get 14 cubic feet of air space and remain under window line.
If you want a good 1000 watt amp check out the JBL 1200.1. You get them off e-bay for $275 and they actually put out 1400 watts. It's about the best bang for the buck going right now.
|