1. Remove the rear-most couple of screws on the chrome door sill. (The panel is attached with the one furthest back.)
  2. Remove the door weatherstrip where it is in contact with the back panel.
  3. CAREFUL: Peel back the leather that is wrapped around the door sill. (This is underneath where the weatherstrip was.) Be very careful here, because the leather will be brittle. I would suggest loading it up with the foot of the neat first, then use a sharp blade to carefully break the seal, then a 6" putty knife to peel it back.
  4. CAREFUL: Next, keep peeling back the leather only, along that same edge that was covered by the weatherstrip, about 1-1 1/2" to expose three screws that attach the wooden part of the panel to the metal. These screws are roughly 1-1/2" from the top, directly across from the center of the speaker "grill," and 2" from the bottom of the panel.
  5. Lift the rear-most end of the panel straight up. There is a 1" square piece of metal screwed into the top rear edge of the panel. It does not need to be removed, just lifted straight up until it clears the hump.
  6. Ta-daaaaa! Your rotted old Infinity Kappa is exposed and ready to be replaced with a brand new Infinity Kappa 52.5i (I got mine for ~$75 on ebay) or other speaker of your choosing!
Thanks to Kent Reynolds