If you really want to get super technical, it is actually illegal for any automobile manufacturer to void your warranty if you modify your car or do not take it to them for service, unless they can prove that the modification, or lack of service is the cause of the said problem.
i.e: Kia, mazda, ford, etc cannot void, say, your powertrain warranty because you installed aftermarket seat heaters.
I actually had this problem once with Volkswagen when they said I no longer had a warranty because I installed an aftermarket sunroof on my New Beetle. Only when I threatened to take them to court did they finally give up on trying to not give me warranty work.
So I hope that helped solve your question
Cheers