I had the 4 position light ( but without the temp. sensor/grill) in my 300SD, as a P.O. upgrade. Same symptoms,went thru the same diagnostic tree. Found that the IC in the light itself was the cause...replaced light fixture...cured. FWIW.

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)