If it was that simple, why didn't GM do it at the factory? They're not really famous for making things more complicated (translation: E$$$$pensive) than necessary. Even putting the voltage monitor on as a short jumper instead of bringing the reference voltage from somewhere downstream has its critics, but I believe that's probably getting a little too picky.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!