Hi Trevor, Cruise was indeed an option on 3.6l XJS both cabriolet and coupe. The earlier system was AE Econocruise but in Feb 1987 was replaced with the more sophisticated Hella ‘computer’ cruise. (Source Skilleter – says vin 108285 roughly)
Same 1987.5 model year revamp they introduced wood veneered ski-slope.

If yours is a 1987 it might be quite straightforward..

The system is however a little more complex than you might think at first sight but I believe should be retrofitable to cars post Feb 1987. I don’t think anyone would recommend the Econocruise version but the Hella by all accounts is a reliable and sophisticated system.

Components:
Ski-slope switch & mounting, ECU which fits under passenger foot board next to main 3.6l engine ECU, brake pedal actuating double switch (and single clutch switch on manual cars), solenoid vacuum valve fits on N/S inside engine bulkhead, Actuator bellows, tubing and linkage to throttle.
I haven’t mentioned wiring or relays either but I think will be already in harness on post 1987 cars. I cannot confirm that but my Sept 1987 certainly has an extra plug in footwell next to location for the cruise ECU.

The 3.6l (& 4.0l) models use a totally different system to that on the V12 cars so no major components are the same (happy to be corrected)

My own cab does not have cruise but I have slowly been gathering the required components myself for future fitting. The ECU latest version is cheap enough at circa £20 brand new off eBay, console switches quite costly as are the bellows as often second hand ones look poor. Double brake pedal switches (cruise and brake light) are more difficult to find but I’m sure if you approached one of the known Jag breakers like ‘JustXJS’ at Derby they could source all the necessary parts.
The parts aren’t really that costly.
The missing ingredient is whether the existing wiring loom has the connections already there and I cannot confirm that myself yet..

If anyone has actually done the modification on a post 1987 I would also be most interested to hear of any pitfalls..

Regards, Robin