I think that people that drive cars are very afraid of having _any_ Microsoft technology in their vehicles.
There is a major stigma that Microsoft needs to overcome if they want consumers to trust their software products in mission-critical applications like vehicles.
Until they demonstrate that their technology is trustworthy, they aren't going to make any inroads in the telematics industry. Both iDrive and AutoPC are perfect examples of this. I doubt the forthcoming "T-Box" is going to be any different.