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I remember seeing a few posts about using the upfitter switch for this purpose, seems to be easy considering most '05 and up trucks have provision for PTO use.
I would like to do this to my truck also. I haul hotshot freight and would like to get into a comfortable truck after being outside in the cold/hot for an hour or so after loading/unloading. The idle only needs to be about 1200 rpm to keep the a/c or heat operating properly.