I'm actually at the dealership right now having mine replaced. The e-brake on the mustang is the most worthless item on the car. It easily sticks due to water damage and corrosion due to the elements. It is a common problem, but if you go under the back left (drivers side) wheel and shake what looks like the bicycle brake cable, it will disengage so you can drive it to a shop.
FYI be sure that you are parked when you do this