Assuming no turn signals have been installed, the brake light switch is the most likely culprit.

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.

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.

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.

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.

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.