Q&A

 Master

Start as you've done by checking power and ground at the driver's rear brake light bulb and working forward. Black is ground, supply a temporary ground to chassis to verify ground is good and get someone to press the brake. Yellow brings power to the lamp unit left rear, dark green wire brings power to the right rear brake light, at a connector at the rear chassis leg, yellow wire continues to another connector in the engine compartment on the firewall on the drivers side (a six wire connector, check continuity on the yellow wire from that point to the rear brake light bulb socket if OK, supply a temporary fused power supply to the yellow wire this connector and see if the brake light works.
There is the 15 Amp fuse STOP-HAZ fuse orange wire at the brake light switch should be live, white wire at brake light switch should be live with the brake light pressed, power from the brake light switch is sent to the turn signal switch where it is sent to the the firewall connector prior to being sent out to the rear connector on the chassis leg and then on to the lamp unit.

 Master

Just to be sure, you did check all the fuses? Most vehicles are fused left and right for many of the front and rear lights. Your third Brake Light could have a defective diode as well as your turn signal switch could have a worn contact. The brake lights usually go through the turn signal switch. Do your turn signals and flashers work properly?

Good Luck!