Ken,
You can mount the compressor in the engine bay, mount the tank in between the frame rails in the rear and also plumb it for regular air fittings to air the tires faster than the stock compressor. I have a 5 gallon air tank that I am going to mount one day along with a compressor and will buy just the horns to add to the system if I ever get the air system done.
This looks like a good system:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=14932&item=5767654 800&rd=1