Hello. The rear brake lights on my '66 2+2 do not turn on when I put my foot on the brake pedal. They both remain dark. However, each turn signal works perfectly when activated. Any suggestions? Tallyho.
Start by inspecting the Brake switch at the pedal. Ensure that it is connected correctly to the linkage. Disconnect the connector and use a VOM to check switch operation. Also check at the connector for voltage.
If the brake switch checks out and power is present, start looking at the turn signal switch. These switches are prone to age problems and wear.
A wiring diagram is an asset if you are not familiar with the Mustang wiring. You may want to invest in one.
There are other items to look for but this is a start.