15" is a big sub to put in there. needs alot of airspace for quality sound. especially two.
with the small cab of the 5th gens, one well built powered 10" or maybe a 12" will do more than enough.
Im running an alpine type -r in a sub wall over smugglers box, and still almost didnt have enough room for the airspace,
15" might be cool to do. but it will either have to be off the wall quite a bit, or not in the box at all...
whatever ya decide to do. post pics. im always up for inspiration. :beer: