2001 GS300 driver side brake light does not work.. Replaced bulb and wiring harness. It is the center bulb. The other 2 lights work fine. Driver side tail light does come on. Used 7443 bulbs from O'Reilly.

 

You need to get a test light or multi-meter and check for 12+ and ground at the plug, one or both of them is missing. Once you know which one is faulty you need to trace the wires back and keep testing them until you find the break. You can poke the test lead into the wire (be careful not to stab yourself). Make sure the brake pedal is depressed when testing so that you know the 12+ wire should be live.