It “can” be opened but Brandon @ MaddRetros looked into it (he has a ‘94) and it turns out that the bulbs in the 3rd brake light are LED from the factory. At least that’s what he said.
“Yes”, you CAN upgrade those but it would require you have to know how to successfully heat the plastic plug off then separate the housing from the lens without cracking or breaking it. FYI, the replacement bulb is actually a strip.
So, end of story: 3rd brake light’s staying as is! This did give me my best chance to buff & polish under the spoiler though ++