I'd think you'd need to reinforce the roof with some fiberglass or something first to take the weight of the amps.... Plus running thick gauge wire up there would be a pain.... How about creating false floors in the rear of the car and mounting them there? (As in cutting portions of the floor out, mount a steel box underneat, and raise the floor in a few inches in the rear).... Never done it but seen it done, looks SWEEET... hehehe
Idea #2, mount them under the rear tray with a motorized rack to either stick out when the trunk is open, or flip up..... (hey its only money right????)
Later
Chris