67 Brake Light Issues

The Green light at the brake light switch should have battery voltage to it. The Green/Red wire goes to the Turn signal switch where it is split to feed both rear brake/turn lights. Check the 10 pin connector at the steering collumn that connects to the Turn switch, verify that all the pins in both sides of the 10 pin connector are fully inserted into the connector, the retainers are prone to breaking and the terminals can pull out. You should find the Green/Red wire from the brake light switch. With a test light connected to ground and the probe inserted into the 10 pin connector at the G/R wire apply the brakes and it whould light up. If not you have an open circuit between the Brake light switch and the 10 pin connector. If it does light then check the Green/Orange and the Orange/Blue wires with the Brakes applied. If the test light does not light on either of these wires with the brakes applied the turn switch is bad.
I hope this helped.

+1 for mustangmutt's diagnosis ....

been sitting here pouring through schematics in the shop manual .... this is quite the messy area as four seprate circuits have to be operated differently depending on what's going on (brakes only, brakes and turn, emerg flash, etc) .... since you indicated that the rear brake lights work for turn indication (if I understood that correctly), then I would think the wiring at the column connector is OK ... seems the problem occurs when only "brake" is activated .... then the culprit would appear to be the turn signal switch as defective in its "no-turn" position ..... but I certainly would do the checks mustangmutt suggested as all of his were spot-on ....

good luck ....