I'm lazy and this is from Crutchfield and if they say it's okay it should be:

Finding the right spot
An amp can be securely installed with just a few screws. But since amps can be sensitive to electrical and motor noise and because interference from the amp may affect your radio reception, try to mount it at least 3 feet away from the receiver. Good locations include:

* Under a seat
Pros: Closer to the receiver, so you can use shorter patch cords and signal cables. Closer to the front speakers, so running wire to them will be easier. No cargo space sacrificed.
Cons: You may have to remove the seat to do the installation. Larger amps won't fit.

* In the trunk or hatch area
Pros: Lots of room for large amps. Near the rear speakers or sub enclosure.
Cons: You sacrifice some cargo space. Longer wires and patch cords required.
Note: When working inside the trunk, tape over the latch, so you won't become trapped inside accidentally. [I found this hilarious but had never thought about it either and am happy I read it]

Keeping your amp cool
Something else to consider when choosing a mounting spot: Amplifiers produce heat, and the heat must be dissipated efficiently. The amp's cooling fins absorb heat and radiate it into the surrounding air. For the cooling fins to operate efficiently, they need a few inches of air space around them. When mounting the amp on a side wall, try to position it so that the slots in the cooling fins are vertical.

Amps should not be mounted on the bottom of a rear deck with the fins facing down because the heat will radiate back up into the amp. Leave yourself enough room on either side of the amp to make all the wire connections and adjust the controls. If you have a subwoofer box in your vehicle, you can mount the amp on the outside of the box.

If you are mounting the amp to the vehicle's floor, check beneath the car to be sure your screws won't puncture a brake or gas line.