On the 1984 300SD,the PCU airflow switch buttons are attached in the same manner as the fan switch buttons (which you have already removed). Detach the wheel assembly (two latches) and the fan switch housing (two latches, again). At this point, release the three plastic latches holding on the face plate of the PCU and pull the face plate out, beginning at the "OFF" button side and moving to the "DEFROST" button side. The white plastic "sub-button" (look again at your fan buttons and you'll see what I mean) is a friction, "click", fit and will give with the right persuasion. The face plate then comes off and you can remove the case top and access the switches themselves (along with their little relays). Rx: some aerosol contact cleaner from Radio Shack. Not resposible for anything mentioned
