Originally Posted by Chromeshadow

AM is correct.
While it's unlikely that running two parallel wires, grounded at the same point on one end and connected to the same positive point on the other, will cause a problem, it's considered a huge error in design. By using two wires you form an antenna where the resonance frequency is unknown. In an entirely DC environment this is usually not a problem, but with all of the RF and pulsed currents in a new car, it is possible that you will create an intermittent problem that will be nearly impossible to track down.