A little correction: RS-232C (and V.24, IIRC) standards use +/- 12V, not +12V and 0V. -12V is called 'mark' and is logical true (or data '1'), +12V is 'space'. Idle data line is on 'space', that is, +12V relative to signal ground. However, there are many other stadard and proprietary serial protocols with different signal levels, polarity etc (e.g. empeg tuner port, which uses inverted TTL levels), so this tells us nothing about OnStar or some other proprietary solution.

Of course, I hope that memory still serves and I didn't mix up something