Where would I be able to find a box that fits behind the seat for a 1990 F150 Reg Cab? I don't really care about what size the sub is, because I'll use what I can find. But, I want dual subs.

 

Make one, I did for my Regular Cab, it fits 2 10'' woofers and a 12'' long amp.

 

How hard is it to make a box for subs? And then more importantly hook up the sub/amp. I have a similar problem with my 91.

If you could possibly point me in the direction of a guide or such that would be awesome.

 

Pretty easy to make a sub box. See if you can find the speaker displacement in cubic inches....If not see if you can find out what volume is needed for each speaker.

I use this site alot: http://www.radiomods.co.nz/speakerboxcalc.html when I'm making speaker boxes, until you run into odd shapes, then its geometry and math time!