locked left rear wheel

restoring 82 380sl . car was drivable when purchased.
finished interior and tried to move car in reverse. car locked in rear on left side. car rode up in air but did not go into reverse.
new calipers were placed all around new rotors in front. rear did not need rotors and also unable to remove them.
took off the tires and able to remove rear rotor left side.
i think the parking brake was locked or frozen tight. No play in the foot pedal in the front on the parking brake and appeats to be released when pulling the nob.
can't get the rotor back on the rear.
help!!!! are the cables shot ( how do i check?) the pads are ok on the brakes how can i close the width of the brake pads to fit inside the rotor? Is this a comman problem? i can't take the car to a mechanic because i can drive it .
Help!!!!!!!!