I have a U. S. spec '91 300-SEL - same as yours except for some rear leg room.
I had to replace the headlight switch a few yrs. ago. The knob with the orange spot in the center pulls straight off.
The new switch did not contain any sort of illumination bulb and it was a dealer item. The knob merely connects to a stem protruding from the headlight switch. I suppose that at one time MB offered lighting for this light switch position. The headlight switch came with a new knob that did not contain the orange spot in the center. I reused the original knob.
Not sure about a "rear" fog light.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.