Unplugging the bulbs on the other side, whether head lights, brake lights, tail lights, etc. Is a quick, easy way to determine if they are involved in the lack of a good ground. If the left bulb is dim, it has to be getting voltage, but some of the 12 volts is being dropped across something undesirable leaving less than 12 volts for the bulb. That undesirable resistance can be other bulbs, resistance in a corroded connection at the socket, or resistance where the ground wire is bolted to the body.
Another way to approach this is to measure the voltages at the bulb. In the unlikely event the highest voltage is less than 12 volts, the problem is in the supply side. More commonly you will find a few volts on the ground wire where there should be 0 volts.
Thursday, March 18th, 2010 AT 9:34 AM