I went the same way that Bob V did. Here is a link to the Senco air compressor that I have installed in the toolbox. I am using a 1500 watt inverter from Cosco.
http://www.juldan.com/juldan.html?tcomp.html
From there it feeds a 5gal. tank mounted under the drivers side.
From the tank I run air supply into a air distributing water proof box with a hinged lid. Inside I have a regulator for the U. S. Gear exhaust brake, a solenoid for the air horns, another solenoid for purging water that may build up in the tank, and a couple spare solenoids for future projects.