I think I can answer that for you. The wiring harness from the dash exits the cab, along with the speedometer cable, through a gap between the toeboard and bottom edge of the firewall just to the left of the steering column. The harness then connects with the dimmer switch just beneath the floorboard. In this area beneath the floorboard the harness connects with the hydraulic brake switch on the master cylinder and sends a wire to the gas sender in the underseat tank. The forward part of the harness goes forward along the frame to below the left headlight. Wires then go to the two headlights from there. The wires to the tail light just follow the same left frame rail back to the light.
There is no harness penetrating the firewall like in later trucks.
Kent
