You've been reading the wrong data, or listening to anecdotal references.
Hyundai, and every other auto maker has specific guide lines about installing radio gear in their vehicles. Not one of them says you can't, and not one of them say anything about warranty cancellation. That fact doesn't stop some dealer personnel from giving you a ration about doing so.
If you want to know how to do it, and do it right, visit my web site. Read the VHF Options article.http://www.k0bg.com