Brainstorming: Try making some sort of adapter for your vacuum cleaner hose and close the other vents and see if you can dislodge or move the blade around any to try and get a better look at it. The adapter could cover the whole vent or use a small one and slide it down the hole. Is it making noise when the defroster is on?

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dtf
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (228,000)
1999 E300 Turbodiesel ( died @ 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles) rusted frame - sold to chop shop
2011 Audi A4 Avant (165,000 miles) Seized engine - donated to Salvation Army
BMW 330 xi 6 speed manual (175,034 miles)
2014 E350 4Matic Wagon 128,000 miles
2018 Dodge Ram 21,000 miles