Why not just place a highly sensitive (-152dBm or better) bluetooth GPS receiver on the rear deck or somewhere else out of sight? With a SirfStarIII or MTK chipset GPS, you shouldn't have any trouble getting a decent signal from any window height location in the vehicle. With my
MTK based i.Trek Z1, I can get a reliable GPS signal in my living room and I don't have any trouble keeping a GPS signal with my GPS sitting in the cup holder of my center console.