First of all, if the power connectors were plugged in backwards the PC would not boot and something would probably fry on the board and render it useless.
Also, it is not "normal" for there to be "shorts in the motherboard". If you have this problem, you'd better get a new board because this is not normal either.
And there is no "short between the motherboard and the case" possible. Motherboards are designed with either no connection to the pads surrounding the holes for the standoff, or else they are tied to ground.
I don't know where you got this information
from, but I wouldn't trust that source anymore if I were you.
The "Buzzing" people experience with MP3Car installations is due to inverter and alternator noise, not a dead PC that won't boot.
Web site: http://www.m2pc.com
Yahoo Pager: m2pc123