how do i change the cabin air filter for 92 w140 300sel?
someone told me there is also an electronic air cleaner inside. is this true?

The car could have three:
1) located in passenger compartment under knee bolster remove panel (cheap foam filter)
2) in heater remove top of heater housing
3) Activated Charcoal (has switch on console) bad news. remove top of heater, remove wiper assembly, remove wiper valley cover, use vacum tool to collapse the seal and pull out.
The Jan/Feb issue of the "star" has detailed instructions on how to do it and what to watch out for Dealer price $1000, parts alone $566