Assuming no turn signals have been installed, the brake light switch is the most likely culprit.
No turn signals installed.
To troubleshoot, move the arm of the switch with your finger to see if the lights come on. If they do, the switch is positioned wrong.
Arm is positioned between firewall and brake pedal bar. It moves as it should when brake pedal is pressed but lights stay off. No amount of back and forth wiggling or anything gets the lights to come on. Spring on switch arm seems pretty weak with a flat spot toward the end of travel.
If the lights do not come on, short the 2 wires on the switch together. If the lights come on now, the switch is bad.
One wire with alligator clips stuck in each female bullet connector does not turn on lights.
If you still don't have lights check for battery voltage on one of the wires. If no battery voltage check the stop light fuse on the firewall. If you have battery voltage the problem is wiring from the switch to the tailights or both bulbs burned out.
One of the switch wires 6.5v-6.7v depending on where I touch the ground. Stoplight fuse is good. I also jumped the fuse with alligator clips and still no lights
If you still don't have lights check the problem is either the headlight switch or wiring between the headlight switch and the stop light fuse on the firewall.
How does the headlight switch play a role in the brake lights? Parking lights and taillights come on at first detent. Headlights and taillights come on at second detent.
