Power is being interrupted; the turn signal switch is a power juction see if the power is flowing thru it.
If you have the same bulb for brake and signal, brake light power needs to be interrupted by the signal lights, The brake circuit has to go through the signal light switch so the problem could be there.
Check for power at brake light switch Check for power and ground at the brake light sockets open wire?
A bad brake light switch or a bad headlight switch, or the power source of the fuse has a problem or break in the wire (circuit).
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.
A brake light switch is a mechanical electric switch that is set in the off position (open circute) and located in front of the brake pedal. When you step on the pedal and its position changes the switch complete's its circute sending power to the brake light. The brake light switch is protected by a fuse or circute breaker and has no other connection with the operation of the brakes.
Check to make sure you have power at the brake light switch, if you have power at the switch, depress the brake pedal, and check for power out of the switch, if no power out with the brake pedal depressed, you need a new brake pedal switch.
I would check the brake light switch first. It's a common problem. The switch is located down on the brake pedal and is easy to change. Use a 12 volt test light to check for power at the switch. You've changed the bulbs or checked for voltage at the brake light sockets first?
The 1996 GMC Yukon brake lights work by receiving power from the brake light relay switch. The brake light relay switch is connected to the brake pedal.
There's continuous power supplied to the circuit. When the switch contacts, it closes the circuit and allows the power to go to the brake lights.
Stuck brake light switch, or a short to power somewhere in the brake light circuit.
the brake light wiring often runs through the turn signal switch i would test power going in to the brake light switch on the pedal and power coming out when pedal is depressed if power is ok suspect turn signal or hazard switch
Replace the switch.