I'm assuming this is an AD truck. If so the power to the stop light comes from a terminal labeled "stop" on the light switch. It then goes through a fuse on the firewall, then to the brake light switch then on to the brake light.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.