This is a time-consuming job. I've recently replaced mine with a DAB one.
You need to drop the rear of the headlining to gain access to the nut under the aerial base.
As the headlining is not flexible, don't strain it.
Now the snowball starts rolling.
The side panels around the little rear windows need removing. It just 'pulls' off, but the ferocious clips will probably break, or worse, the panel itself will break around the clip mountings. I got away with only one not holding, and the stiff panel holds OK, but there could be tears. That also needs removing the panels that cover the rear door shuts, meaning the rear seats have to come out. That was a fight, but had me laughing. The bolts come out easily enough, but once detached, if like me you have the split rear seat, you now have a heavy, awkward assembly with a hinge in the middle. Best to have help. I did it on my own, but it was a bit like trying to hold a defiant cat.
Now you can detach the door seals at the top, and remove the grab handles, to allow the rear half of the roof lining to drop enough to get at the aerial nut.
If you are replacing it with an OE replacement, the rest is easy. If an aftermarket one, the car's cable has an odd connector on it, so you'll need to run a new 'normal' cable through to the front.
Now that means removing the door tread panels on the nearside front and the glovebox.
This is all from memory, and it is the sort of experience that one's memory tries to blank out, so I might have missed something. Start early, and have all day, best the whole weekend. It'll cause tempers to fray, and tears to be shed, but when finished there is the satisfaction of a job well done.
Whilst fitting mine I also ran a new power cable from the electric aerial output of the head unit to power the amplifier in the aerial, and with DAB it meant two aerial cables, one FM and one DAB.
This was all after I'd spent some time painting the new aerial white, as that was how the original was, being an Alessi. Prat or martyr? You decide.
Good luck.
Where are you? I may be able to help, point and advise, or hand you the tissues, if you are not far away.
