Being 6ft tall and keeping the seat all the way back has eaten most of the room in my standard cab. For now all i have is speakers plus a set of 6.5 components in the kick panels, I think they're qlogic kick panels got them many moons ago. MTX used to make a box for behind the seat that would fit a 10" sub, and if i had room i'd consider maybe on of those self contained low profile under seat sub boxes.
It's not installed yet (will be in a couple of months) but my system consists of: a Pioneer DEH-80PRS head unit, two ol' school RF Punch 75HD amps, a Punch 5-1/4" comp. set in the kick panels, and one Punch 10" sub. Deceptively simple...
I have x2 Pioneer 10” TSSW2502S4 Slim subwoofers with an Kicker 43DXA5001 D-Series pushing them in boxes that I made to fit behind my seat. My seat is one click from all the way back.
View attachment 340418
if you want to go less expensive but compact and still decent bump do the combined amp and slim sub. I got one from Walmart for about 90 bucks and its the brand Blakunt or something. works great and doesn't take any space at all. I didn't drill it down just wedged it between the seat and floor so it doesn't shift and vibrate.