For what it's worth, I have both the GPS and the Sirus/XM antennas in that stock location under the defroster grille on the right side of the dash. The GPS never loses lock and I'm satisfied with the performance of the XM radio with the antenna there. It performs just as well as it did in my Blazer with the antenna outside the car.
In other words, I don't think it's your antenna's location.