Radioranger,
It appears GM was issued a patent on January 25, 2011 for the very thing you are talking about, U.S. Patent #7,874,610
link below
Patent Images
The view the above at
United States Patent and Trademark Office you will have to download the software to see the images of the patent document. It is free.
Here is a better link at google patents:
http://www.google.nl/patents/US7874610
Bondo