[my first mustang]

I had this same problem over 38 years ago on my 71 Roadrunner. I've also had the same thing happen on my antique 35 Dodge 15 years ago(a brake drum on the back of the transmission)so it doesn't matter how old or new the car is. All you need is a little water or slush to get in the cable and the emergency brake will freeze. I learned not to use the emergency brake and just leave the car in gear when the temp. gets below freezing. Driving the car before the emergency brake releases can cause damage to the rear brakes.