My car has one but never had Sirius and the antenna is in the rear glass. That's why I was unsure of its purpose because it seems to have no function in my case. Would a fin be installed if there was no reason to have it, maybe as a standard item?
For mass production purposes, it's possible that every example of a certain model gets the shark fin antenna, even if some examples do not have the feature that uses the antenna.
For mass production purposes, it's possible that every example of a certain model gets the shark fin antenna, even if some examples do not have the feature that uses the antenna.