Can I clean the evaporator in Mercedes W124 cars through center vent on the dash?
Is there a wide open path from the center vent to the evaporator so that a foam can go through when the ignition is switched off?

1z (einszett) Klima-Cleaner instruction goes like this.

Turn off ignition.
Spray the foam to center vent on the dash.

They say the foam reaches the evaporator.

In order for this method to work, there should be open path from the center vent to the evaporator. On my 1992 300D, however, I hear some flipping sound when I turn off the ignition. If this sound indicates that the path from the center vent to the evaporator is now blocked, I will waste my Klima-Cleaner.

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.