Problem with 1995 E300D radio antenna
 

Good day all.

A while back the antenna stopped retracting although I could hear the motor spinning after switching off the ignition.

I got a replacement antenna mast from Ebay. Now the problem is that the nut holding the mast turns endlessly but doesn't come off.

Fix anyone?

Sounds like you need to pull back / remove the trunk lining that is covering the antenna, and see what's loose. I think the hollow shaft from the antenna motor/whip housing is aluminum (fragile) so a gentle approach is the order of the day. If you have a small strap wrench, maybe you could counter-hold the shaft with that so you can loosen the nut?

Could be that someone "repaired" the antenna before, now you've got to deal with their mess.

When I realized that the nut would not come off, I removed the entire unit and again tried to unscrew the nut. It simply turns but does not come off the shaft.

Well, in your place I think I would open up the cover and see what is loose inside. Be ready for the coiled up antenna nylon tail. I don't think it will go "sproing!" when you remove the cover, but it is under tension. I've been into an '84 201 antenna, was a bit of a hassle to get it all back together so it worked but not impossible if you're patient. Not sure, but you need to think about the clock timing of the wheel that coils up the antenna tail, so you can maintain orientation so the antenna knows what is up and what is down? Hopefully someone with expertise will chime in here soon. Worst case scenario is that you break this one even worse than it already is, and have to get a replacement from a salvage yard.