My 85 30E has a dimmer, and so does my 90 300D. There are two knobs protruding from your instrument cluster. The one on the right sets the clock and the one on the left resets the odometer to zero when you push it in. That same knob controls the potentiometor that sets the dash illumination for the dash. Turn it ALL THE WAY to the right. It may act like it doesn't want to turn, but turn it left, then right , and it will almost certainly brighten your lights.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"