Im also looking at putting a sub into my VY II Storm. Have orderd Phreddy's kits and when they arrive will start building the system up. And being a ute you aren't going to get 12" or 15" subs in there, and Im interested in adding some bass not SPL ( I spend upwards of 4hrs a day in it :whistling )
Still thinking about where Im going to put the sub and how to build it. Space is really the issue with the utes and as far as I can see there is only two real options. A box down in behind the drivers seat (passenger seat well has the jacks and is good safe storage for laptops etc). Or as a few others have done on here get into the well beside the fuel tank in the tray and put a box in there. Each has its pros and cons
Behind the drivers seat, mmm box has a max volume of about 16 litres and is shallow, because of the sloping front needed to fit with the back of the seat. To give you an idea take a volley ball (8” diameter give or take) put it behind the seat and push the seat fully back, just as the seat gets to max travel it starts compressing the ball. The well is also only ~280mm high, so a 10” shallow sub is the best Im going to get in there using a sealed box. Maybe a pair of 8” side by side or in an isobaric enclosure using a piece of 225 mm PVC pipe. That said behind the seat is hidden, good sound travel and realativly eaiy to take out (ok you got to take the seat out).
Beside the Fuel tank, first thing that springs to mind is what happens in a side impact crash. Ok that aside this solution allows a good 10” sub to be used and has a reasonable box volume. Not sure how much sound actually gets through the airvents into the cabin though. I have seen one installation where linings behind the passenger seat well (where jack handle is) were removed to open up the vents to help this. Its a major job to fit and remove, even more so when you have a tray liner installed. Design wise probally stick to some variant of a sealed box
Still weighing up the options though might go for the first one to start with as its reasonably easy to do.