So after some time looking at the original fixed antenna that was all rusty and crappy looking I decided to do something about it. Not to mention it had a really hard time receiving anything.
I crawled under the car to remove everything: cable, ground plane and mast mount.
The antenna itself was just rust at this point and the coax cable was less a cable and more like a elongated water canister (which gave me a good shower while I was under there).

I looked at the prices for the replacements parts and it just didn't sound good with me.

Moral of the story I scrapped everything except mast mount and ground plane.
I hollowed out the mast mount with a 8mm drill bit, shaved the bottom part with the angle grinder to remove all the old rubber and 3d printed a sleeve in Nylon.
In the Nylon sleeved I threaded an accordingly shaped M6 threaded rod, added two nuts and chamfered the edges of the rod, added some shrink tubing for protection and this is the final result: a universal antenna mount where I can mount any antenna I want (FM or CB)

I will make an SMA or N connector adapter so I can connect either my FM radio or my CB radio and have fun with it.
Big plus is that I can unscrew it easily like a modern antenna for storage under a cover during winter

I will post readings from my SWR meter when I finalise the assembly.


How it looks installed

The modified mast mount

 

I 3D printed a plug and deleted the antenna entirely. I'd glass it over if I ever had to repair that part of the car.

An SMA connector is a really clever idea! I'm looking forward to the updates.

 

Can you post some pictures of the plug? I wonder how it looks! Also probably next weekend I will be able to post updates!

 

Here's what I could find. I have more photos somewhere. Perhaps I'll find them, too.

And do click on the link in the quote. When I looked into this years ago, a WiFi or GPS antenna would have been somewhat useful. Now, phones are so good that any external antenna is pointless. The next antenna that goes in any of my cars will likely be a Starlink Mini.