Travis, I am not aware of a 4 wheel disc setup that uses the rear calipers for the parking brake. If you look where the parking brake cables enter the wheel backing plate, you should see an adjustment hole for the starwheel. If I am wrong, then I have learned something about older 300D's. Most MBZ's have 4 wheel disc setup. Most have little brake shoes inside the rear hubs for parking, emergency brake.
1987 BMW 325