I have 2013 plate Mx5 Ventura with no cruise control fitted to steering wheel . I was wondering if I could buy the switch off steering wheel and then I would anyone with good knowledge help me understand if it’s possible
If it’s of any interest a couple of years ago I investigated the possibility of retro fitting cruise control to my 2010 Mk 3.5 PRHT. Ideally I wanted what would have been fitted by the manufacturer including the steering wheel controls. I approached two main dealers asking whether it could be done and had a zero response from both. Literally that - no reply of any sort. A couple of independents advised that it couldn’t be done but without, I think, giving it any real consideration. Eventually I went to Freelance Mazda in Chatham (where my car is serviced) and Mark there made a real effort to explore the possibility. In short his conclusion was that it was theoretically possible but that the cost would be prohibitive as apart from the cost of the cruse control module there is no existing wiring and a complete new steering wheel would be required not just the switch unit. I realise that your car could be a Mk 3.75 but suspect it wouldn’t be any different.
I temporarily abandoned the idea but later went to a competent local auto electrician who supplied and fitted a cruise control module with a steering column stalk control (image attached hopefully). They made a very neat job of it and it functions perfectly. Just so as you are aware of what you are contemplating it set me back £550. A lot for something that is used relatively infrequently.
That aftermarket option looks a great idea 
the stalk pictured looks good and a lot of cars have a stalk rather than steering wheel controls. Just for info, the actual OEM steering wheels are the same under the skin. Blanking plates can be swapped out for switches but as said, where do you go from there if no wiring in place after the switch.
Barrie