The fin is for Onstar and WiFi I believe. I can add a WiFi plan to the van with unlimited data for $25 a month. I’m on AT&T and can add it to my existing plan for $20 a month. My van also is capable of using the MyChevrolet app that lets you lock and unlock the doors, blow the horn and see diagnostics. You can also schedule service and other things as well.

 

Wifi antenna is probably inside. It would not work well from the wrong side of sheet metal.

 

It’s the cellular antenna. Yes, the WiFi antenna is somewhere inside.

 

cellular + GPS

 

My cell phone works fine inside my '06 van with no fin on top😳

 

but Onstar doesn't

 

I have no idea what the star thingy is, so I don't need it.

 

I don't remember asking, but ok. We are all very aware that cell phones will work in any car.

 

I'm just trying to find out why it's there. Is a add on option?
It looks like it would get knocked off in the 6'9" parking structures.

 

I already explained why it's there. It provides a cellular link for the Onstar help and car phone, and satellite link for navigation and other locations services.
Onstar provides a number of things like emergency services, navigation, diagnostics and other things. You can find out all about it on their website: onstar.com

Because the roof is curved, the tip of the antenna actually sits at the same height as the tallest point on the van.