My truck is a regular cab with a bench seat, so I don't have a ton of room for much gear. As far as I can tell my only two options are under the middle portion of the bench seat, or on the back wall of the cab beside the box for my 10" sub.
I was leaning toward putting it under the seat because I thought it would stay in place a bit better and might be less likely to pull the screws out of the sheet metal.
Any suggestions on which would be better?

I was leaning toward putting it under the seat because I thought it would stay in place a bit better and might be less likely to pull the screws out of the sheet metal.
Any suggestions on which would be better?
