Rep Power: 0

Here's what I have:
- Kenwood KAC8402 amp (60w x 4, 180w x 2)
- Infinity Components front (90w RMS)
- Sony Xplods rear (60w RMS)
- Polk Audio DB104 sub (270w RMS)
- Alpine CDA-9857 head unit (18w x 4)

Q1: Cooling
I've had the Kenwood amp for about a year now and after about 2 or 3 songs full blast my sound lowers, which i assume is from over heating, so i lower the volume and after a few minutes it'll go back up again.

The amp is mounted on the back of the rear seat nothing under it. I was wondering what the best way to keep it cool is. I'm thinking about putting washers under it to raise it off the carpeting on the back of the seats. Also I've read about putting computer fans on it. Where would i place the fan(s)?

Q2: Setup
I just received an sub to put in my trunk, without buying another amp for now what is the best way to set it up? The head unit has special sub controls so i want to use that instead of using the tri-way option on the amp.

Is it possible to bridge the head units power to use on the rear speakers? I'm thinking this is a good way to do it and still maintain the fade controls instead of hooking both pairs up to the same amp channels.

So the main question is, what is the best way to have 4 speakers + sub on the equipment I currently have?