Just in case anyone's interested in how to get to the aerial (plenty of video guides out there, but they all seem to involve bending / damaging the headlining) you need to drop the headlining so you can actually get to the aerial mount.

Luckily it's a very elegant, simple process on the 500 so nothing to worry about :)

I didn't bother taking off the windscreen pillars for a full headlining removal because you don't need to for this job, but it'd no doubt be quite easy too.

Firstly, remove your tiny parcel shelf, and put the rear seats down. Pull off the rubber insulator around the top of the doors and tailgate, taking note of how it fits back in, and the felt strip around the sunroof (if fitted). Then look for one pozidrive screw just beneath the parcel shelf bracket, and another one where the rear pillar trim meets the side trim:


Once those are out there are just 4 push-in clips holding it on, so removing the trim will just involve a bit of gentle pulling, firstly from around where the bracket is and then near the window.

Next pull up the bit of trim over the front seatbelt upper mount, and undo the 17mm bolt holding the mount on, being careful not to lose anything. Pull the central door pillar trim away from the bottom, and it'll unhook from the top. The fasteners / hooks holding it on look like this just so you know where to pull:


Next carefully position a trim tool (or something else clean and suitably shaped) around the coat hooks in the rear, and gently turn them 1/4 turn so the rectangular mount disengages, and then just pull them out. Not much force involved here, so if you're needing to apply a lot of pressure you're turning them the wrong way, or have turned them too far. The clip design looks like this so you get some idea of how far to turn them:


Once that's out, remove the round plastic fastener near the tailgate opening, repeat all steps on the other side, and you'll be able to carefully pull the roof lining down. Full removal would no doubt just involve removing the windscreen pillar trims, but I didn't need to do this. Refitting is of course a reverse of removal; once you've done it once you should be able to get the roof lining in or out in about 30 mins, I reckon, which is the quickest headlining removal of any car I've ever worked on :)

Once the headlining's dropped you'll be able to see the mounting bolt for the aerial (10mm in my case), and the aerial connector (Fakra 2 on mine), though these differ according to year, so you'll need to remove the aerial to check before ordering a replacement. Aerial will just wiggle out once the mounting bolt's removed, and from there you can either fit an OEM replacement, attempt to fix the no-doubt corroded-and-useless passive amplifier under the steel cover, or try and replace it with a shark fin and active amplifier (which is my plan):


Once the aerial was out I did a quick bit of measurement, just about enough room for fitting a small amplified sharkfin if you've got a sunroof, though finding an aerial with a female Fakra 2 fitting is tricky, so I've just ordered one with the standard ISO aerial plug, and an ISO->Fakra converter separately.

Tricky to find Fakra 2 socket fitting:

20mm square (not round) hole where the aerial fits.