Shark fin antenna repair?

Look at post #9 in this thread: Someone stole the “fin” from the roof of my Chevy Volt 2017

The antenna simply unplugs and installs with a single nut. The two challenges you will have are:

• You may have to remove the ceiling lighting fixtures and/or drop the headliner in order to get to the nut. (Dropping the headliner is not easy and it something that needs to be worked around carefully as to not damage it before it goes back up against the roof.
• Paint. You may have to take the new antenna to a paint shop to get it color-matched to your car. Some paint shops may require you to leave your entire car with them so they can do the match right. (And guarantee the color will match.) Painting may also cost you more than the entire antenna.

If you can find a used antenna from a junk yard that matches your car, that may save you a ton of money, particularly on paint.

As an update to this, I managed to replace the antenna. Here's how I did it.

I bought an antenna from a wrecked Volt online, it was about $100. It was grey and my Volt is blue, so I unscrewed the casings and switched them around.

I decided that it was too risky to try to drop the headliner and opted to go inthrough the rear dome light. Removing that thing was a huge pain in the ass. The outer casing was a pain because it's attached by 3 clips in one side and I still don't quite understand how it is actually fixed to the body because the clips didn't really make sense. Then the dome light itself is clipped into the roof. Its the same dome light as on a few other Chevy vehicles, namely the Silverado and I found a great guide on another model's forum with photos. You have to pry the clips out of the roof casing with a pry tool or a couple of screwdrivers and I found it to be pretty much impossible to do alone because you can't see a damn thing up there.
Once the light was out I could see the bolt where the antenna was attached but it was quite hard to get to, you can use a socket wrench but quite tough and tutoring to position. The bigger hassle is unplugging the two cables attached to the antenna, you better have tiny or hands or know a kid willing to stick his hands into an area with lots of wires because it is so hard to reach. But I managed to do it and managed to reattach the new antenna in the same way, but I can't stress how frustrating and difficult it was.

Now my antenna is on but it isn't flush to the roof, so my last question for the forum is any advice on how to get this thing flush? It is screwed in all the way. See photos.

Thanks to @Kelly2221 for posting this thread. This problem got me this week... I backed out of my garage with no idea my door stopped just at my roof line. All was clear out of my rear view mirror but the shark fin caught the garage door and "BAM!".

I think this is a "thing" for the Volt. Looking at other GM cars, our fin seems to stick out a lot higher on the roof.

This creates a very frustrating situation and repair. The factory procedure to service the roof and antenna is to remove the headliner, which involves taking out the A, B, and C pillar trim, disarming the airbags, and replacing single-use hardware associated with the air bag tethers. It must be a $2000 job to have a dealer do this.

I have been following the same procedure @Kelly2221 did. My antenna faired well but the clear plastic retainer cracked - so I need to get a new antenna just for that part. It would be helpful if we had squirrel-sized hands for this job, but we don't I was able to push through it with a 1/4" ratchet and a deep 10mm socket. Removing the two rear headliner retainers near the hatch also helped.