About 10 years ago I had a Vauxhall Vectra 2ltr CDi which I wanted to retrofit cruise control. People, including Vauxhall said it couldn't be done. I then found on the Vauxhall forum that as long as I had a clutch switch fitted, which I had, then all I needed to do was replace my standard indicator stalk with one fitted with the cruise control. I was supplied with a part number and the price which was £37. I fitted it and low and behold it worked perfectly.
The logic given was that with building mass produced cars, all the ECU's and other parts are all the same but if that particular model wasn't to have cruise, they simply fitted a standard stalk, it all made very good sense to me.
I just wonder, that if a model like a 3ltr X250 which can be specified with cruise can be retro fitted in the same way.