Welcome to the forum.

From all the W140 restorations I have done. Any Antenna repair I have done the whole assembly has to come out from inside. The grey paneling comes out. The Audio amp along with the holding bracket, and you will see the black antenna assembly.

The decorative cap snap in from the outside, and grips the barrel rubber gasket at the top. You will need to wind-up the antenna from the inside along the big plastic gear. The plastic Stub on the big gear will break that hold that single turn spring. I've never bought a new gear but fixed the stub with a bolt

There are two flavors of these antennas. The early till mid year cars have everything enclosed in the black plastic housing. The mid to 99 cars have the electronics separate to the mechanical parts like this..

If your antenna stops moving - Mercedes-Benz Forum

The later cars suffer from corrosion problems due to the barrel rubber gasket at the top of the antenna getting hard.

The coupe W140 cars have a black rubber decorative cap, but here is a exploded diagram that may help.

https://www.k6jrf.com/MB_Hirsh.html

Here is a how too

http://www.benzworld.org/forums/w140-s-class/1650277-antenna-rebuild.html


Hopefully this helps too,

Antenna not working - Mercedes-Benz Forum

Martin