So my cruise control would not hold and I replaced the two turn signal
bulbs with genuine German made bulbs.

The bulbs in there were 1156 bulbs with brass base.

I had spare German bulbs out of a spare car I had.

My cruise appears to work.

So I am a believer, now - where to I purchase a supply of
correct bulbs for that car ?

W123 1980 300D.

Seems everyone sells Sylvania 1156/1157.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250