later 90-up trucks have a nice plastic tray that lays behind the seat to hold stuff.
stock speakers hang on brackets in the corners unless you go with truck box speakers.
myself I don't keep anything back behind the seat. I don't keep any heavy/loose stuff in the cab...I rolled a truck one time and a glass 16oz MtDew bottle hit me in the head about 3x,I needed stitches in all 3 places. I seen it hit me 2x for sure.
good thing my 12x16 metal tool box was in the back of bed at that time. I had been riding it in the pass side floor earlier that day.
so strap/screw/bolt it down in the cab to be safe.