It should only effect it when the cage is grounded, but your defroster grid may be seperated by a fraction of the wavelength of the frequency that the GPS uses. I'm new to the GPS thing so I don't know what freq that is. In any event, if that were the case, the defroster grid would be acting as if it were an antenna and you can get a standing wave on it even without any connections. That would cause enough signal chage to lose data. Or, another possibility is that the wires connecting the grid may not be the only ground. Remember that the whole car is a ground, so if the grid intersects metal anywhere, it is grounded.
2006 Chevy Colorado: VIA M10000 EDEN, 1 GB RAM, 80 GB 2.5" Seagate HDD, USB Slim Slot DVD/RW, Holux GPS, MobileVU 10.4" LCD (touch not working yet), VOOMPC Case (blue), 70W DC-DC supply.