The W123 Mercedes Hirschmann antenna is a Bear..
 

The W 123Mercedes Hirschmann antenna is a kluge!
I had three which I hoped to resurrect but the insides were so rusty and gummed that I replaced mine with one from a 95 SAAB 9000. The fit and appearance are pretty good.
Even if I had fixed my Hirschmann I would have been afraid to use it, because the thing belongs in a Museum!!

The whole episode makes me feel guilty because I am trying to keep my 300D mint and original.

Most used Hirschmans are found in better condition and can be taken apart and cleaned, restoring them to usefulness. Sitting in a junkyard with the trunk open will make anything rust. Usually the antenna is packed full of grease, which keeps a lot of water out. But there are always bad ones and it sounds like you found them.

I think they are sorta cool (over-engineered, but cool). I've had mine apart a few times and they both work fine at the moment (but I'm sure I'll have them apart again). If it's rusty, I would just buy a new one and replace it (but I would miss playing with the old ones).