How have you guys put speakers in your vehicles? I have a 1990 f150, and want to put some subs in, but theres no room under the back seat.

 

There is plenty of room, you just have to utilize it correctly. Custom built box, shallow mount subs. You don't need a huge system unless your goal is for everyone to know your coming. I've got a single 10" tucked behind my passenger seat in a single cab and it puts out more than the cab of the truck needs, I downgraded from a 12" because it was just too much. I've heard that 8"s actually sound better with the small amount of space in these cabs.

My box is only 12"hX12"wX(5 1/2" deep at it's thickest point, 3" at it's smallest) or something close to that.

 

A single sub would work for me. It would have to be out in the open though. In my truck, there is 0 room under the rear seat. I couldnt even fit a sub woth no box under it.

 

The shallow mount subs only require less than 4" of mounting mounting depth, this one on crutchfield is under 3".
http://www.crutchfield.com/p_130TSS2...-SW2002D2.html

I usually use crutchfield to read reviews and select a few options for speakers than google the model numbers and find the best deals online. The one I listed can be used in a free air application, i.e. mounted in the cab wall or seat back/bottom. I've seen some pretty sweet setups in camaros/firebirds when they don't want to give up the trunk space or the back seat, they mount the subs to the seat frame: cut out the foam/seatcover where they want them, then sew/staple/glue the cover down and attach the sub so you can't see the hack job the did to the seatcover.