I'm going to install my subwoofer amp in the rear cargo area. How do I secure it? I hesitate to use sheet metal screws, because the gas tank is right below.

 

Lots of people mount them to the box or to a piece of wood that is covered in the same material as the carpet. You may be surprised how well it would stay in place. Mine is mounted in place of the spare tire and I pay for AAA membership that gets me free flatbed towing for the first 75 miles.