electrical help

The most likely cause of the problems listed are due to a failed multi-function switch. The multi-function switch (turn signals switch) is what controls the turn signals and high and low beams. The Mustang headlamp switch or its plug-in connections are also notorious for failure and more so when the car was equipped with fog lamps. The brake lights are on their own circuit, separate of the turn signals. Brake lights are powered at all times, even with the ignition switch off. Power should come from the battery to the brake lamp fuse, go to the brake lamp switch near the brake pedal and to the brake lamps at the rear of the car. The bulbs may also be burned out, corroded, or otherwise failed and not illuminating. The audio system may have been incorrectly installed and the installer compromised the lighting circuits at fault. If you plan to diagnosis and repair the problems yourself, I recommend researching the Internet for wiring diagrams of the circuits your are troubleshooting. The Fox Mustangs are popular enthusiast cars and there is much information on them on the Internet. Always start at the battery and work to the to the other end of the circuit (ground). Somewhere between those two points the problem will be found.