Located at the upper end of the brake pedal almost under the dash
you need to replace the brake light switch that is located above the brake pedal underneath the dasboard Here's a link to a video that 1aauto.com has that shows you how to replace the brake light switch.
Replace brake light switch.
The brake light is controlled by a relay switch. If the relay switch is bad the brake lights will stay on. The brake light switch simply plugs in and out. Unplug the brake light switch until you can replace it.
Adjust or replace the brake light switch.
This is usually cause by a stuck brake light switch on the brake pedal. Replace it.
replace brake light switch located on brake pedal under dash
The brake light switch is bad, look on top of the brake pedal under the dash and you will see the switch, replace it and you will have brake lights.
Check the petal and make sure that the plastic part that touches the brake light switch when the petal is in contact with the brake light switch is still on the petal, if it is missing you will need to replace it, so the petal will make correct contact with the brake light switch.
APPLY BRAKES AND SEE IF BRAKE LIGHTS COME ON. IF NO BRAKE LIGHTS, REPLACE THE BRAKE LIGHT SWITCH LOCATED ON THE BRAKE PEDAL ARM AND THIS SHOULD ALLOW YOU TO SHIFT OUT OF PARK. IF THAT BRAKE LIGHT SWITCH IS BROKE, YOU WON'T BE ABLE TO GET IT OUT OF PARK
The brake light switch might not be connected properly. Remove the brake light switch for 10 seconds and replace it to its original position.
How the brake lights work on a 66 Mustang is very simple. Power goes from the headlight switch to the brake light switch, then through the turn signal switch and back to the brake lights. There is no fuse in that system. There is a self resetting circuit breaker in the headlight switch, but, if that was tripped, none of the lights on the car would work. The first thing to check is the brake light switch itself by bypassing it. Unplug the connector from the switch and run a jumper from from one slot in the connector to the other. If the brake lights come on, replace the brake light switch. If they don't, the problem is either a wire has lost continuity or there is a problem in the turn signal switch.
Probably a brake light switch.