Don't have much to add other than my 2007 Mazdaspeed3 also has this unused connector. I've had the car since new, so there's no question about removed parts, etc. Probably something available on the Grand Touring/higher spec models.

I'm still salty about Spotify quietly removing support for my Pioneer head unit (along with others) shortly after the Car Thing came out. Luckily I took the hint and cancelled my subscription and haven't looked back.

Where's Goldbug? I thought he turns up just about everywhere?