I've read that the W126 tail light design was self cleaning. What does that mean and how does it passively work to self clean itself?
Yep, very clever design. Since dirt and snow would only get trapped on the protruding part of the taillight, the inner part would remain clean. Obviously the Mercs of today,even though technologically very advanced, don`t have the innovations of yesteryear....
As said by MBeige, the aerodynamic flow keeps around 50% of the taillights clean and visible during bad weather.