Not really recommended if it's freezing cold, but try pouring hot water on the rotors.
Then, turn the engine on, but turn off the traction control off. Move the car forward and backward. Even with the brakes on, you can force the car to move a bit if only one of the rear rotor is engaged. The water will freeze solid quick so you may want to pour hot water frequently.
Then, turn the engine on, but turn off the traction control off. Move the car forward and backward. Even with the brakes on, you can force the car to move a bit if only one of the rear rotor is engaged. The water will freeze solid quick so you may want to pour hot water frequently.