You will have to jack up and support the car, remove the rear wheels, remove the rear caliper's and brake discs, inspect the parking brake shoes, perhaps they are totally worn out or the spring that keeps the shoes retracted has broken and is causing your problem. Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info.
and get an online repair manual subscription. Alldata are great for information, he information is year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online