the diagram I posted above is scanned from the Chev shop manual, the wire from the starter [violet] goes directly and only to the ignition switch ... a factory wired system will NOT start when you hit the foot switch unless the key is on - I doubt GMCs were different considering that by 55 wire colors were industry standard

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics