Previous owner did something here and I can’t unscrew it. It just spins with a plier. Any ideas on how to remove it so I can stick a stubby?

 

Instead of pliers, try a pair of vice grip. Keep consistent outward pressure as you turn

 

^^ thats what i would try first. Looks like some homemade stuff. When you get it off and the thread are ok you can go to a stubby version.

https://www.ecstuning.com/Mini-2007-Cooper-R56-S-Coupe-L4_1.6L_N14B16A/Exterior/Antennas/

 

An impact wrench and a 6 pack of your beer of choice can solve almost any issue.

 

looks like the remains of some aftermarket antenna. Try using visegrips on the square part and turn while pulling up.

 

Be careful with this one. There is a threaded hollow collar inside the shark base. When you are turning the outside antenna- you are also spinning the collar inside. If you pull up you run the risk of ruining antenna base. If you poke around the shaft base under the black rubber you will see it. This needs to be wedged before you attempt to unscrew the top.

 

I was jkng about the impact but perhaps some penetrating lubricant is in order?

 

The threaded insert that the antenna screws into is just mounted into the plastic "shark base"...if you use too much force the insert will just spin within the "shark base". I had a similar issue with a poor quality aftermarket antenna (not purchased from a NAM vendor) many years ago. I ended up paying the dealer to replace the entire base....can't remember the cost...seems like it was between $300 and $400 for parts and labor. The dealer had to partially remove the headliner.

Maybe you'll have better luck than I did...