The issue is the radio signal can't get though the metal roof of your car.
If you could put it into a plastic dome on the outside of your car so its GPS signals aren't blacked that would be ideal!
We both know thats not possible when the antenna is part of the unit. Depending on what you have you might be able to use this: [https://buy.garmin.com/en-US/US/p/1326|GA 25MCX Remote GPS Antenna (Low Profile)] which allows you to place the head unit at a more comfortable location while you drive and have the antenna mounted either at the back of the mirror so its less blocked or externally.