I know I am late on this thread, but I m working on a cruise control for my K1200S and will be on my soon delivered R1200GSA. I chose the Rostra electronic cruise control to avoid the need for a vacuum accumulator for a vacuum actuator. The K1200S and the R1200GS have a connector near the front of the bike for connection to the GPS accessory. This connector has three wires:
Brown = ground (pin 1)
Blue/Green = TAA (pin 2)
Red/White = switched power (pin 3)
I will be hanging a scope off of the TAA lead next weekend, but I am fairly sure it is a conditioned speed pulse from the ZFE (chassis control module). This would be used by a GPS employing dead reckoning, not currently used by BMW. The Rostra unit also looks at an engine tach pulse and disengages the cruise control in the event of throttle runaway.
The Rostra has all of the electronics and a throttle servo in one module and can be purchased for about $200. Here
is a link to Rostra controls website.