Accessing Rear Heat Ducts

[rjfjr1]

This may sound rediculous, but I've been hearing a rattle under the pass. seat upon change in momentum. For a variety of reasons, including daughter's constant frustration at completing the colors of the rainbow. I suspect a Crayola marker has become lodged in the heating ductwork under the seat.

Any clues on how to access the duct? I can remove the plastic fascia, but this gives minimal incremental access. Seems like I need to get in to the meat of it, remove seat, remove carpet etc. Any other ideas? I've read the posts about front suspension rattles and I'm fairly certain the Crayola is at fault. I'm pretty sure it's the green one.