If the motor does work, you'll hear it going bizzzzz bizzzzz when you have someone turn the radio off and on. If you hear that, that's the motor trying to extend the antenna up and down.

If that's not working, on 2 jap cars I had, it was just a plastic cord of sorts, that hooks up to the antenna and forces it up and down, no big deal. I'm assuming you can buy a generic one on ebay for $20 or so, or maybe find a MB one used. I'm assuming you'll get raked over the coals if you buy new.

1991 300d, 90k