Here is some info from the Oasis Off Road Compressor site
http://www.oasisoffroad.com/p3.html
They make penumatic compressors that are built from York air conditioning compressors. They run up to 200psig (less than an A/C system). Using a 12 volt DC series wound motor, the puppy sucks about 180 amps. The unit is 62 lbs and is 20 inches long. It uses a 5 hp motor!!!

A helicopter usually uses 400 Hz, 3phase motors......I doubt that it would be a DC motor. 400cycle motors are much smaller for equivalent horsepower but few road vehicles have 400Hz power!

the good old method is the simplest, cheapest, and most reliable.
Putting the evaporator behind the seat is a viable idea that should work. Some argue that the a/c blast should be hitting you in the face but???? I still like the idea of a/c in the seat cushion and backs.......and then have a little periscope outlet to cool the family treasures......