ooh i say !
only jokin m8
using the spare wheel well is a superb way to minimize the amount of space taken up . u need to strengthen it up a fair bit using fibre glass and ideally mdf strips , but the wheel well has only just under 2 cubic foot of useable air space , so you needa sub that has been designed to work in a small enclosure .
you,l also need to remove the fuel cutoff from the well as the vibes directly inside it will trip it .
if you want the sub to think its in a slightly larger enclosure it might be worth thinkin about running it magnet out !!
one last thing , subwoofers were never designed to run flat , which means you will need slightly more power to keep controll over it properly (compared to a woofer in the upright position)