You're right there is tie down straps for the spare it has been awhile since I read the manual.

 

Went to my owners man. (on cd), says the straps are to hold (reg./full size) tire off veh., down in the spare tire well.
Evidently a full size tire won`t fit in the spare tire spot and clamp down snugly.
So, what do people who put speakers and amps in the cargo space do...?

 

I built a rack that sits on the spare tire and is held down by the big plastic nut that normally holds the tray and spare down.