Of course, mounting it on the sub enclosure (if you have one) requires NOT drilling holes into the chassis...which is always preferred...but..
under the pass. seat seems ok, and yes, a fan would help...
If you want a fan, it's pretty easy:
get like a 3" 12-volt fan from radio shack and a relay that will handle 12 volts
tie the relay into the + and the remote wire....
it acts as a "switch", so that when you turn the deck on it supplies the remote lead tho the amp AND the fan the full 12V.....
I had an installer do mine, though......