All brake lights work except for the 3rd back light in the back window. I have changed out the bulb 2 times and have had a mechanic fix it temporarly. It lasted 2 months but now it doesn't work at all. Checked all fuses. I have been told the wiring harness needs to be replaced or soldered correctly. Been told the wiring harness goes to the tail light. So the problem has to between the back window and tail light . It maybe one of the wires connecting to the bulb or all...not sure. Which connection of the 4 wires (black) connecting to the bulb area do I have to reconnect? I have taking apart the 3rd brake light system in the back window and not sure how to get to the wiring under neath the dash. Not sure what the mechanic did to make it work temporarily. I moved out of state and working on fixing it myself. How do I get to the harness underneath the back window dash to fix or replace?

Any ideas out there or suggestions?