Hi Chris,
Just to be sure, you have gone in to the settings menu on the high line screen first by selecting SET, then from there you’ve pressed and held down the MENU button for a good 10 - 15 seconds and it hasn’t changed to the service mode screen?
It should do this on all high line units. On tourers it always seems to be the sharks fin that is the problem, not so on the saloon which has the antenna under the parcel shelf and so is protected from the elements.
The only other thing I can think of is that the connector on the antenna cable going to the back of the nav computer is damaged and so not making contact. I’ve got this on one of my cars at the moment, it’s the end with the 90 degree silver plug on it. I need to either cut the end off and splice it to a donor plug or replace the entire cable run to the parcel shelf.