Keyed power source

I have a 2007 GMC Sierra Denali 2WD. I am looking for a convenient 12 volt, keyed power source under the bed to mount an Air Lift compressor for the air bags. I have looks at wiring diagrams and they are not much help. Since the main fuse box in the engine compartment appears the same as in the Suburban type vehicles, and has a factory leveling option, I thought that would be a likely source except that it is hot all the time. Needs to be a keyed source in the event of a line, bag failure so the compressor does not run until the battery dies. The compressor system keeps the preset minimum pressure on start up. My thought is that the wiring harness is the same in the trucks and that there might be an unused wire under the bed that I can utilize. I have not looked into tapping into the dash fuse panel yet until I rule out and easier, accessible source. Any suggestions would be appreciated.