Remove the passenger front carpeting and take a look-the duct that the rear seat vent plugs into is right there, close to centered below the front seat. I'm pretty sure if you remove the rear duct (held in place w/ 1 screw) then you'll be able to wiggle the front duct out from under the seat where it passes through the chassis. If this doesn't work, pull the front seat (4 bolts & 2-3 electrical connections) and go from there.

Good luck!

__________________
"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo