Check this out. A Ford tech laid this on me.
It seems Garmin and some other GPS brands have a problem with the built-in technology in today's automobiles. The electronics in your vehicles are are active whether or not you use them, causing crossed signals with satellite communications. Your vehicle is more powerful than your dash mounted GPS and therefore results in false reports to your screen. If you buy a vehicle with built-in GPS, even if you don't use it, it will conflict with your hand-held GPS.
Sounds reasonable to me. I don't think he was pulling my leg. My truck doesn't have all that fancy stuff and my Garmin 57 LMT works great. Maybe the auto manufacturers want you to only use their equipment. Something to think about.:h