Thanks for the heads up on Plenums Busboy
I've been meaning to get around to checking mine so I've just done them.
It looks pretty clean & dry in there except for a few leaves.
I used a stick with double sided tape stuck to the bottom to pick those big leaves up, it works a treat
I rodded the drain with a length of bicycle brake cable then blew out the remainder with the air line, I also blew out the side drains too.
Edit: It seems not before time too! the missus has just taken the car out of the garage & there's a mound of leaves on the floor where I rodded the plenum.