I took the wheel off last night. Found a human error that kept the cable from turning. The upper cable wasn't lined up properly to the lambda counter. Unfortunately I think I've had that apart since the speedo last quit so I can't prove it's the source of the problem.
The root of the alignment problem seems to have been the way my footwell was arranged. My wiper controller mount has been hanging since I bought the car, and I think some of the wires were pulling on the speedometer cable. I just found where the mount belongs in another thread and I'm working to clean it all up.