They normally have a ground on each side but GM has a very bizarre way of wiring their daytime head lights. I'm doing this from memory for all other cars.
When there's a break in the ground wire on the right side, current wants to go through the low beam filament but can't find a complete path to ground. It finds an alternate path by doubling back through the high beam filament, then over to the high beam filament on the left side, then through its ground wire. That's why I told you to remove the left bulb. That will remove that alternate ground path and the dim right bulb will go out.
You can troubleshoot the problem either way. With the left bulb removed, you will find 12 volts on all three wires wires on the right side. With the left bulb installed, you'll only have 12 volts on the low beam feed wire, (or just the high beam wire if that's what's turned on). The high beam and ground wires will have around 6 - 9 volts. If you can't tell which wire is the ground wire, remove the right bulb, then measure the three voltages. One wire will have 12 volts when the low beams are turned on. One will have 12 volts with the high beams turned on. The last one is the ground wire.
Wednesday, November 21st, 2012 AT 7:31 PM