My unit is an add on so my experience may not count...
Originally I had the antenna located on the dash, near the windshield, which seemed like a good location, with a clear shot at the sky. I would get frequent dropouts and poor signal.
I relocated the antenna to the roof of the truck, above the third brake light. It made all the difference in the world. If I get a drop out now, it is due to a very reasonable condition (tunnel, etc. ).
I would guess that the factory installed radio does not have the antenna in the best location.