I'd really like to find an amp that fit under the front seat. I use my cargo area with the rear seats folded down frequently, and I frequently carry 4 passengers. So that rules out the cargo area and under the rear seats. My navigation brains are under the drivers seat, so this leaves under the front passenger seat as my only option. My seats are manual, I imagine power seats have less room under them. I might eventually put a shallow-mount sub under the rear seat, drivers side.
Mounting one inside the tailgate is a pretty interesting idea. Running the power wire from the battery all the way back there wouldn't be a problem. But seeing how there is only one way in to the tailgate, and therefore one way out of the tailgate, how would you run your speaker wires from the amp to the speakers without running them parallel with to your power wire?
Running them parallel to your power wire would create massive interference. You're not supposed to cross your power wire with the speaker wires, and if you do have to coss it, you're supposed to do so at 90 degree angles. Running wires to and from the tailgate, your speaker wires would be parallel to the power wire for at least 2-3 feet.
The Blaupunkt THA475PnP is the only amp I've found that looks remotely close to fitting under a front seat. Here's a link to the product page:
http://www.blaupunkt.com/us/7607792130_main.asp