Thanks for the wiper grill advice. I had already removed that a few weeks after I bought it to remove some paper I saw in there. I didn't see any clogs at that time, but I really wasn't looking too hard down the plenum either.

I'm certain I got it all with the Shop-Vac dryer vent attachment, I ran it all the way to the top, then for good measure when that was done reversed it and blew it out as well.