Unfortunately I don't know exactly what part number to tell you. You can however, easily make any switch with a 3/8"(?) round barrel mount work.I just tried to figure out why the washer wasn't working in mine...and the switch is busted in two pieces. What are the popular options here? Switches at NAPA weren't inspiring.
You want to look for a panel mount SPST NO momentary switch of your choice, push button or toggle . I changed mine to push button from a big ugly toggle switch. Ideally it would have a pair of 1/4 male spade connectors on the back that would match the original so you could just plug it in as a direct replacement. I don't know what switch that would be though.
This one has screw connectors, so if you want to leave the GMC harness unmolested you'd have to make two short adapter wires to go from the screw terminal to a male 1/4 spade connector. Then just push the males into the female connector. They should stay but if you're worried about them coming out you could wrap it in tape. Or, if you're not concerned with modifying the GMC harness, cut the connector off the harness and solder on connectors that fit the switch.