I fitted all the cruise control bits to my 850 a couple of weeks ago but it didn't work. Yesterday I investigated and found that there were two reasons for this, both related to the donor car being an automatic S70 and not a manual 850.
The pull rod (25,25,26) was telescopic, not the hinged arrangement you see here. Even with the bellows fully retracted it didn't start to pull the pedal. It needs to be telescopic or hinged so you can operate the accelerator without compressing the bellows, so my initial thought of gluing it to a fixed length wouldn't have worked.
When I fitted the parts I was pleasantly surprised to find that the brackets (27 and 29) were already in place. I should have looked closer, they are quite different.
With the S70 brackets in place the pull rod was able to work properly and I took it out for another test drive. It still didn't work, which brings me to the other difference.
With the donor being an automatic it didn't have a clutch valve. The brake and clutch valves (12 and 15, they are identical) work in two ways. When the plunger is released it opens a valve that lets air in to discharge the vacuum pressure, and it also opens a switch to cancel the cruise. Once I'd thought about it I realised all I had to do was bridge the contacts on the clutch valve plug.
That got the cruise control working, so all I have to do now is buy another brake/cluch valve and some vacuum connections and it'll be a proper job.
With the exception of grubbing around under the steering column, which is never fun, it was all surprisingly easy to do. All the electrical connections and mounting points are already there.