a lot of people don't hook up the subwoofer when they install an aftermarket radio because it requires an extra step of a relay. The factory radio drives the sub amp directly, but aftermarket radios cannot. You need a relay. Or I did like Oliver suggested and I just connected the subwoofer to switched power and it comes on with the car.
Yes the Fiero cabin is small and a new suwoofer is far better than nothing but don't get your hopes up too high, it's still a 5 1/4" woofer. Not going to rattle the windows.
[This message has been edited by jscott1 (edited 04-24-2014).]