When the trucks were built (47-53 anyway)There were only 2 wires going to the rear light(s). One for tail and one for brake, fixtures were grounded. Small bulb for tail and large bulb for brake, both single fillament.
When after market directionals were added, additional wires were run to the 4 light assemblies that were installed .
If you removed the add on directional lights and didn't change the rear wiring harness the brake lights are still connected together.
