Well, if you hear rubbing I would check my brake pads. Low brake pads shouldn't cause the light.
If the fluid is correct, then I would also check the sensor. I assume (big assumption) that when the car is running in neutral and no emergency brake, that the light is on. You should be able to disconnect the sensor and jump the terminals to simulate a closed contact, thereby clearing the light. Remove the jumper and see if the light comes back on. Without the jumper, apply brake pedal to confirm that the brake pedal being pressed clears the light. If the above is true then I would guess you have a bad sensor.
If the above isn't true, check the emergency brake. If the mustang (and other cars) are like the 4Runner, there is a sensor mounted under the brake handle such that when the handle is lifted (brake applied) no contact is made and light goes on. When the handle is down (no brake), the contact is made and the light goes off. The sensor is nothing more than a plunger. I believe this to be the only sensor on the E-brake system since it is strictly mechanical.
Write back after you check it out.