Well it would be best to install them where they can get some air, most I guess go in the trunk, and as far as fire I've never seen one that hot, usally there is a thermal potection for that on the amp, unless it shorts out somehow!
yeah make sure its in a well ventalated place. under a seat is the place most people put them, or in the trunk. just make sure its either its in a place with lots of air around it, or in a seal place with a fan. My soundstream vga600.2 is in my tool kit in the back of my durango and i have to build a case with fans to keep it cool. Otherwise it would shut off after 30 mins because of heat
There is sufficient air flow under a seat? I will only be running a 90x2 RMS 2 channel amp so it is nothing huge. But I was thinking that you aren't getting much of a flow of air under a seat to maintain ambient temperature surrounding the casing.