I lucked out.
When the Sprinter was brand new and we were testing everything,
my son was standing at the left rear when I tried out the rear fog lamp.
"Yep, it works" he said.
It was weeks later that I read in this group about the one sided nature of this European system.
Had my son been more diligent,
I would have been back at the now defunct dealership squacking the bum right rear fog light.
Last week I thought the real one on the left had burned out or otherwise failed.
We were leaving on a trip, and while testing the lights on our much narrower trailer just for the he11 of it
I pulled on the fog.
No joy.
No big deal, who needs it anyways.
How many U.S. drivers following me would even know what it means if I did encounter fog, remembered to turn it on, and was driving slower than them?
Trip went great, but today I played around with it a bit.
At engine off, fog light on, it flashes like a turn signal!
Engine on, nothing.
Same son figured we should check it with the rear lighted step substituted for the trailer hitch unplugged from the harness, and BINGO!
Don't know if it was ever mentioned here before or not,
but for good reason I guess the lamp is disabled when a trailer wiring harness is being used.