Whats behind the carpet?..
#1
#2
Of course, general rule of thumb is to look before you leap. I have done many in depth audio systems for the 97-03 f-150 so I can tell you that there is vitually nothing on the back wall of the truck except that backpad of the seat. As far as the floor goes you might run into a wire depending on what side of the supercab you are going into. Just remember, look before you leap!
#4
Depends on where your talking about. About 3-4 inches down from the rear window there is a hump that helps support the back seat pad, but below that is just a thin sheetmetal wall that leads to the outside. It is a very bumpy and uneven surface, maybe to lend a little to structural rigidity, but it is just a panel of sheetmetal and that is it.