I just, a few days ago, plugged in a new antenna from EBAY and it, the GPS system, immediately began working properly. The original antena is somewhere up top, under the roof cap. I did not attempt putting the new antenna up there. In stead, I tried placing it just under the front dash (as close as I could get it the underside of the fiberglass dash) and it works well there. I was located in some pretty heavy tree cover at the time so I think I'm good.
Just give a few simple locations a try, you may be surprised to see it work well.
Mine is a Kenwood DNX5080EX system.
2010 WBGO Vista 32k - sold/traded
2014 Tiffin Allegro 36LA