Note: nearly ten years after I sold the 944, I still get a fair number of web visitors. I ran across the original servo and its controller in the basement the other day, so I thought I'd post some updated commentary. The original technical content is from 2003.
Here are the pinouts for the servo and original controller.
I attempted to build a digital replacement for the mostly-analog tempostat circuitry. The schematic diagram of my replacement board is available. It used a PIC16C765 for low-speed USB 1.1 connectivity. In retrospect, this was a bad choice for several reasons. The 'C7x5 series uses a UV-erasable EPROM (remember those?), and the available bandwidth for low-speed USB was less than a high-speed RS-232 link. A Cypress EZ-USB would have provided higher-speed transfers, and I could have used a serial EEPROM for code storage.
Here are the original servo board pin assignments. The idea was to use the 40-pin 16C765 for development, and drop in a smaller OTP 16C745 for the final design.
And here is a diagram of the cruise control mechanism, courtesy of Pelican Parts (servo is part 5).
Note that this entire discussion is based on the early 1985 model year. The 1985/2 and later have a two-part connector on the controller box, which might be very different than this one.