That red light behind your 1995 Mustang's steering wheel that illuminates when you brake is most likely the
Brake System Warning Light. It indicates a problem with your braking system. This could be anything from low brake fluid to a malfunctioning brake light switch, a problem with the ABS system (if equipped), or even a failing brake component.
You should not drive your car until you have this checked by a mechanic. Driving with a brake warning light illuminated is extremely dangerous.