I could be wrong but the clear 3rd brake lights were an option when new, and are no longer available Oem or aftermarket. New red light replacements are available however.
I started off the fix thinking I could replace whatever was inside the light with 12v red LED tape, cut to length. Which I am sure would also work. Soldering the new resistors into place was fiddly, but I tried that first. I poked the (new) resistor wires under the old resistors then bent the wires back on themselves, which made soldering easier. New resistors cost cents, and almost certainly have a higher watt rating than the much smaller originals which look like they have burnt out and/or melted.
In NZ I was told there is a rule that 75% of the LEDs must work to pass the annual test so I did not want any more to fail.