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)