[IanM]

I'd guess that the aerial is in the rear screen for the coupe, but you can't avoid the "crappy" aerial in the convertible - they can't use the rear screen for obvious reasons and the fin probably isn't suitable for standard radio.

It might explain why there is conflicting information about which options require the fin. Maybe the better reception of the (I prefer "old school") aerial over the rear window one means that the fin isn't necessary for some options on the convertible but it is with the coupe.

However I know sweet fanny adams about aerials, so apologies if this makes no sense!