gokartkid - If you had the buttons added and the system activated, first of all, probably most dealers would never know it didn't come that way. And second of all, the only warranty that would be voided would be warranty related to adding that system. However, a dealer will try to void rear brake warranty (for example) over adding the cruise like you want to do, but the Magnusson-Moss Act prevents them from doing so. Unless a consumer knows that, the dealer will take advantage. I'm sure it happens all day every day.
I personally think your risk level to add the OEM switch and program it is very close to zero. Good look with it in any event and congrats on your 2017!
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View my fuel log 2020 Mirage ES 1.2 manual: 42.4 mpg (US) ... 18.0 km/L ... 5.5 L/100 km ... 51.0 mpg (Imp)