Ok, I got two new Sylvania bulbs and got to work.
Nothing to it but it was a ***** before I got finished.
For one thing, the insulation on the wires to the lamp socket was brittle and started flaking off; liquid tape to the rescue.
Also I dropped first a metal clip, then a bulb, and "down in there somewhere" it went. When will I ever learn? I finally fished everything out with a magnet. Then I could not get the drivers' side to click back into place. Finally I removed the coolant reservoir and got a better grip, then got it in there.
Finished at last, and both low beams worked. No more messages.
Dangit.

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Providing a home for these cars:
1951 Buick Special De Luxe
1977 Rolls-Royce Silver Wraith II
1970 Rolls-Royce Silver Shadow
1986 Mercedes-Benz 420 SEL
2005 Mercedes-Benz E 320 CDI
2012 GMC Sierra 1500 4X4
1927 Pierce-Arrow Series 80
1931 Pierce-Arrow Series 43
1926 Ford Model T coupe