How do you loosen a seized parking brake cable?

yOU HAVE TO FIGURE OUT WHICH CABLE IS BROKEN (LEFT OR RIGHT) BY PULLING OFF THE BRAKE LEVER COVER (sorry about the caps lock)

The bar across the top of the lever is the cable connection keeper. High end is the broken cable.

You have to remove the rear tire and brake drum to change the broken cable. It is torqued to amazing tightness so I'd suggest a local garage for this if you are a novice. It has to go in from the wheel side, up the tube, out into the levelr area, turn up around the lever and into the keeper. It'll drive you nuts and get REALLY greasy.

Also a good time to go ahead and get the rear brakes serviced. Make sure they turn the drums smooth on a lathe and use new cotter pins on the big axle nut. If you do not see a silver cotter pin bent over the big nut make them change it.