You've presumably removed the RH boot cardboard liner, and can see lots of wires and stuff. Take photos. Disconnect the wires and earth strap attached to the aerial, and remove some nuts and washers under the mounting plinth. As far as I recall, fiddly, and it's easy to drop them, so it's worth clearing and cleaning your boot before hand so you can find them again.
If your's is like mine, which is also a Facelift from 91 (the 99th made, according to the VIN), the main retainer left is a chromed nut in the chromed assembly that passes through the wing. AFAIR it's got two small flats for a spanner and is about 1" deep; it can be resistant to turning, and can turn the aerial assemble instead of coming undone. Indeed, you probably should remove it before the nuts under the plinth.
The aerial motor assembly comes apart quite obviously, and will be full of dirt that's washed down the aerial shaft, and possibly water, and old grease, and the aluminium tube that aligned things will have rotted at the base. I found a replacement brass tube at my local model shop (now closed), and had to rebuild the electronic timer using bits from Maplin (now closed), so if mine stopped working again I'd probably have to buy a new car
In the past I've used a replacement short flexible rubber mast from Halfords - they even had a near matching colour. They don't seem to sell them any more. Here's
a link to a similar one on Amazon.
Not tried an internal one.