It's the little yellow bulb here....
It comes on independently or usually with the parking lights, so in dense Euro cities with plenty of streetlights you have a visibility glow without blasting people with headlights. They light up the housing itself without projecting much light outwards. It's starting to become rare since we have ubiquitous LED dots and stripes all over cars now.
I have them in the 7" round Cibie's in my Alfa, but I haven't wired them in yet.






