Is there water in your trunk? I highly doubt the amp is getting wet, it'* mounted a foot off the bottom of the trunk on the back seat, and its covered. The amp is easy enough to disconnect, I would try disconnecting it when you shut the car off and see if that fixes your problem. Your mobile installer may have wired it to constantly be on.
You can see the amp on the left, it says monsoon on it. There is a plastic panel covering the amp, it is held in by magnets, and I think one screw on nut, possibly more. Just pull both the wiring harnesses out when you park it, and see if the battery stops draining.