I have my 1987MBZ 560SEC DASH apart AND the Under-Hood Wiper-Assembly cuz I noticed the BLACK FILTH in the Cabin Air Vents. It seems that there is NO filter under the wiper assembly? or not anymore... (I will check pre-blower) but that suggestion... Painter's Monkey "DUST" MASK... is pure genius... and I didn't think of it! I am fashioning filters now.
I will repost with pictures l8r. U can also use pillow stuffing and screening to fashion filters for vents, ducts, and almost anywhere in the air system... watch temp, if you are in the engine compartment!!!
Upgrade, prevention, leave it better than you found it, fix everything (that the other guy f'd up)