| RE: Antenna Mast
Assuming that the aftermarket antenna mast is correct, there is a small gear inside the mechanism that tells the motor when to stop. It might simply be loose.
If it's intact, you have a wide variety of masts to choose from if you try Autohaus AZ. I got the $30 one, last item, flat top peg with "h" for Hirschmann.
http://autohausaz.com/search/productx.aspx?sid=lj0w2ky1thj5u1eupqjnzr45&makeid= 800016@Mercedes&modelid=1193778@300DT&year=1983&ci d=13@Body,%20Trim%20%26%20Lighting&gid=1332@Antenn a | 
