A few weeks ago I picked a complete cruise control system out of a 1986 Jetta at the junk yard. Including "cover charge" (the junk yard charges $2 entrance) and two new clips from the dealer the whole thing came to just about $50. Today I had the time to install it in my '87 GTi. Easy work, but time consuming. Installing actuator and pump under the hood and pulling the wires and hoses took only an hour or two, but installing the wiring etc. in the cabin was tedious.
Anyway, the thing works great, wish I had done it years ago.
Thanks to Ingo Braune for info on speed sensors.