The guys at mobile one said that they can mount an amp (a big on at that) behind the panel near the rear seats. They said that they mounted a navigation system there for someone.

No one has said anything about this on this board. Does this sound right?

Which panel would that be again??

Amps need some degree of air to allow for proper cooling, I'd imagine the air back there is rather limited. Maybe install a fan to move the air across the fins.