id say that sort of thing yes but usually theyre added to the dash or similar, and most are only starter buttons rather than a full on 'on/off' set up which would be very involved and basically work in a similar manner to an old fashioned cut off switch (remember those from before the days of immobilisers?) used to.
even if it could be done with canbus systems (I'm sure it can be) it would be incredibly difficult, complicated, and probably very unreliable.
a simple 'Start' button, however, would be considerably easier to do, id imagine
I have seen one on a you tube vid fitted to a very nice late model w163, the ign barrel and key were done away with and replaced by an illuminated button and a couple of remotes ,chap actually used them to remote start the car.. sounds a bit naf I know but looked like a quality bit of kit. I imagine you would need to be a rocket scientist to fit it though:?.
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