Make sure you store your amp in an area in the car that stays cool so it doesn't overheat in the summer. Good locations include:
On the firewall (passenger side).
Pros: Short wires and patch cords required. You won't have to remove a seat or climb into the trunk.
Cons: Only very small amps fit here. Closer to some common noise sources.
In the trunk or hatch area.
Pros: Plenty of room for large amps. Near the rear speakers.
Cons: You sacrifice some cargo space. Longer wires and patch cords required. If working in the trunk, tape over the latch, so you won't get trapped inside accidentally.
Under a seat.
Pros: Closer to the receiver, so you can use shorter patch cables and signal cables, which are less prone to noise and signal degradation.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. Warning: removing your seat could deactivate your SRS system. Larger amps might not fit. You must elevate the amp to avoid contact with water from rain or snow brought in on passengers' shoes.