Tony, the stalk interface measures the voltage at the socket. This happens to correspond with measuring the resistance of a pure resistor such as the Sony stalk because of the internal resistor network. If an external source provides a non-resistive voltage input, then that voltage will be measured, and the internal resistor network will have virtuall no effect (it's now merely a heater). See the circuit diagram on RioCar.Org for details.

Snowwhite, Tony's advice to actually measure the voltage present at the input is good - remember that your stalk's output will have its own source resistance which may have an effect (and if the resistance is different for different outputs, it may cause some buttons to be indistinguishable). And get your hands on an oscilloscope if you can, to check whether it's using some serial protocol.

The stalk interface needs its input to be stable for a certain time (Hugo? Patrick? anyone?) before you get the packet at the tty device. Perhaps your stalk isn't holding the line steady enough? That's quite likely if it's using some digital protocol.