Original 1979 GMC & Chevrolet Dealer Electrical Wiring Diagram Service Manual. This very informative Factory Manual covers Medium & Heavy Duty Truck Models. These detailed Wiring Diagrams are what the Dealership used to diagnose and repair your GMC. Very useful and rare since they were not offered to the public. Good condition with normal wear and soiling from use in a Dealership [see photos].  X-7939, ST-352-79A