Originally Posted by stretch05
All lights work/operate properly except left brake light. Just replaced the tail Light fixture... So it's wiring somewhere correct?
Where so I start?
Downstream. Start at the back and work your way towards the front and follow the wiring. This will also confirm if the brake unit is working correctly too. The first thing you can check is the connector plug that plugs into the tail-wiring of the brake light assembly.
You can get a simple electrical tester at any auto store for like $5. You just use the needle-end to prod the wire terminal (the hot) and use the alligator clip attached on any grounding surface. When you press on the brake pedal, the LED light inside the tester should illuminate, this will tell you if there is power coming through that particular wire.
To test for shorts to ground / continuity check, (a break in the ground wire), you'll need a multimeter with a resistance read-out. But start with the simple power test first, it's most likely a break in the power wire somewhere up-stream. Do it in sections a couple feet at a time, as long as the tester is not lighting up, that means that section of wire is not transmitting the proper voltage. Keep going section by section and when the tester finally illuminates, replace the wire back to the brake unit at that point.
If by any chance you get the tester light to come on at the most-downstream part, (the connector that plugs into the back tail-wiring of the brake unit), there is a problem with the brake unit itself. Take a piece of wood and wedge it in between the pedal and driver seat so that the pedal is always pressed in, that way you have continuous power being applied through the wiring and don't have to go back and forth or have someone else press the pedal constantly.