Most of the time it's the control switches. They oxidize, and no contactie!! I have diligently taken them apart, and clean the contacts with rough paper. Not sandpaper, but just paper. A fingernail file will work, but only drag it backwards maybe twice. That silver is very thin. Occasionaly, rapid switch operation for 100 times might clear oxidization. (desperate measure) Physically clean it is the best, or replace it.
You might fetch the online manual, and find the schematic for that switch, and figure out how to resistance test it, or use a powered test light.
The next thing would be a burned up, dirty motor, or bad connections somewhere.
If the compressor runs using the switch that works the air hose [For pumping up tires] and you have air pressure at that point the compressor is working.. If using the increase switch isn't getting air to the shock it could be a cracked or broken hose.. Or if you're lucky a lose connection....
Had the same problem with my 99' 1500SE this summer while we were on vacation. Found out that the air line was leaking at the fitting that the auxiliary hose screws onto. Cut the fitting off, replaced and reclamped and everything is fine.