You can get an 8" shallow mount sub in the back corner of an 80 series...it's good also to look at the WHOLE HOG bracket for amp setting, if you don't know about it.Do you have speaker and amp part numbers?
My Tacoma has a Rockford bass box, it's great, I'm going to measure and see if it'll fit in the back corner panel under the plastic molding. I've seen a few post to guys sneaking a subwoofer box back there.