On my 49-state '88 300TE, that spot's dead. I'm thinking some sort of errant light is flickering back there? What happens when you turn the key to "on", but don't start the car? Are all the others working as they should??
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car