Well it had to happen sooner or later...
I fitted my cruise control today:
YouTube - Cruise Control (Alfa 156)
Went pretty smoothly from start to finish - attached pictures show some details of the design. I used pieces from the '80s - old-school Technic which pre-dates the car by many years! I think with modern pieces (e.g. studless beams as used in my Mindstorms NXT 2.0) you could create something more elegant, but I didn't have those pieces spare, and also, some of the geometry to mount the microswitches (there is no glue or tape used) didn't seem to be possible without using old-style toothed couplings. These aren't available any more, so I have no idea what the total project cost would be
It was about $20 for the switches.
The on/off switch is simply a round rocker switch mounted in the column shroud - I made the hole using a holesaw that comes with a reverse sensor kit.
For wiring, I used small spade terminals ground down slightly to fit in the original connector. Pin numbers are on the outsides of the connector.
Pin 1 - Common supply to all four switches
Pin 2 - Not connected
Pin 3 - On/off switch (i.e. connected to pin 1 when 'on')
Pin 4 - Decrease speed (backwards rotation of the wheel)
Pin 5 - Increase speed (forwards rotation of the wheel)
Pin 6 - Resume (lifting of the bar)
It works better than you might expect thanks to the use of precision microswitches. The bar moves 3mm and the double-wheel rotates forwards and backwards by a few degrees with a nice spring-loaded, tactile click. Bracing on the back of the 'stalk' ensures that it cannot come apart, and it is fastened to the column shroud by three screws to give appropriate rigidity.
I'm interested to know if anyone has come up with their own designs and I'll be keen to see improvements on my design (other than Alfa Romeo's $240 stalk that probably doesn't use microswitches