Sure enough it was the rheostat on the rear of the dimmer post. As a temorary fix I soldered a jumper across the back of the rheostat, instant fix.

I do not, however, advise doing this on the side of the road, at night; after you run a diesel out of fuel beacause the fuel gauge was not 100% accurate....

Lesson learned

1981 500SEL 182,000 kilometers
1982 300SD 267,000 miles