Hi victorlll. Welcome to the forum. All of these lights share a common ground wire. When that wire is broken, current will still go through one bulb and find an alternate path to ground through the other bulbs. That makes them all light up but not at full brightness.
You can prove the wire is broken by removing the left backup bulb and the all of the bulbs on the right side. That will remove the alternate paths to ground and the left brake light will not work. If the left brake light still lights up but is very dim, current is likely going through the two license plate lamps to ground.
The black wire from each socket is the one to test for continuity to ground.
Thursday, September 9th, 2010 AT 2:40 AM