If it's not the bulb or the ground wire, check the connections. If all else fails, it's the turn signal switch.
Check the fuses.
What fuse is for the 2005 Ford mustang Rear Brake lights?
I'm guessing you're talking about your turn signal light. From what you wrote I think I'd have to say it sounds like a bad turn signal switch. The brake lights run through the turn signal switch on mostly all vehicles.
The most common cause for brake lights to be out on a 2001 Mustang GT is a blown bulb. There are some instances where there is faulty wiring.
i have a 1992 F-150 all of my lights work but my rear passenger side brake lite and signal do not i have changed bulbs and checked all my fuses and wires what do i do?
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.
the turn signal switch is probably bad
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 the fuses
That the lights need fixing maybe?? That the lights need fixing maybe??
the muscatel middle school! (^^)