I would suspect the starter - where the battery connects to the starter [big terminal] there should be another wire that goes into the harness to the cab, disconnect that wire and the one from the side terminal into the harness so that the only thing possibly still connected to the battery is the starter .... if this is a foot start, the box on top with the button that gets pushed may be defective

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