I thot BMW's parking light is activated by the indicator stalk??? This knob method is how the parking light is activated in the Merc.
I guess too many ppl don't know about this activation method via the indicator stalk.. As I recall, E46, E36 and even E30 has indicator stalk-activated parking light.
And yes, parking light will stay on when it's activated until you deactivate it or until the battery runs out... It's ok to leave it on overnight, it won't drain much power. But don't be a 'blonde' to leave it on when you go for 3 weeks of overseas trip...