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

Try above on buy parts.

__________________
83 SD

84 CD

Autozone has them...

make sure they are the correct size and wattage. Someone in my cars past put to high of wattage and it deformed the reflector..I had to break the bulb out and file the plastic down so the right sized bulb would fit.

Junkyard or dealer if scarce. Although used are perhaps not the ideal replacements. Great for testing though once you establish the filaments are still good. .
Nice to know there is some truth in the stories about the tail light bulbs especially.
There now should be a rush of members checking their tail light bulbs for original part numbers. . Not that there are that many cruise controls out there not functioning properly. I thought I might have had that market cornered myself.

Believe what you want but it is still a complete myth.

The turn signals have nothing to do with the cruise control. So, whatever problem corrected itself will certainly return in the future.

Can you explain why?

The cruise control only looks for a resistive ground or positive voltage. The bulb type, size, wattage, brand, etc. has no affect on it.

Exactly.

Perfect example:

My dad's SD has the brake light mod (mine does too) with 4x 21watt brake lights + a LED third light at the top that uses another 4-5 watts....and his cruise works perfectly.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

buy form this site

True, the wattage does not matter. What does is if the brass base type is corroded and no ground is available for the cruise when putting the brakes on.

Plated bulbs are still better as they provide operating tail, brake, and turn signal lighting more reliably!!

__________________
1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150