Today's problem, not building air pressure 07 VT830

Today go to play with the big toy, started the engine and as normal the air compressor kicks in to build up air, but this time it just kept running. I notice that after 10 to 20 secs it normally turns off, but not this time. I can see (the grass moving) and feel air coming from what I believe is the air compressor located behind the fuel tank on the drivers side. I never had a problem since I picked up the truck early this month, but yesterday, in search of a short for something else, I found that the wire harness to the air thingy had totally detached (corrode apart, see pictures) and today it's not building pressure.

Can someone tell me what this is and where is the wire harness leading to so I can repair it.

Thanks in advance.

 

Yes it is the air dryer, and do anybody have a diagram of route of the wire harness.

 

Need the purge valve replaced, thanks for the help