Fine folks of the Phaeton forum:
I have an update on the speakers, and also a PSA for the good of all members.
Upon removing the door panel (what a job!) I tested the contacts directly on the speaker for an open circuit. To my dismay, the circuit was, in fact, open, meaning the speaker was toast. Not being one to quit easily, I decided to toy with the speaker. I pressed on the speaker (driver?) while touching the contact points and I got a beep, indicating a closed circuit. I pressed hard for a few seconds (be careful if you attempt this, it takes some finesse and you must press the right place, mine was near the upper-left of the speaker) and got a solid tone. I have since reconnected the amplifier and reassembled the car (excepting the door panel) and tested the speaker and voila! I now have a working speaker!
As far as I can tell, something was shaken loose inside the speaker. I choose to attribute this to the remarkable door slamming that has tortured my Phaeton.
To all Phaeton owners: Feel justified when you cringe as someone slams your door -- they might ruin your speakers!
I am hoping this is the same problem with the other two bass speakers, I will post my findings tomorrow when I have the energy to remove the other two door panels.
Thanks so much for your help guys, I couldn't have done it without you!