I once bought a GPS repeater. It consists of an antenna and a magnet to put on the roof of the car, a cable and a battery pack and a little transmitter and an internal antenna. The idea is to have the internal antenna re-radiate the signal in case you use a GPS receiver inside a car which has metal-shielded windows or for other reasons block the signal.

I've never found out how well it works, if at all, because even my old Sirf star II receiver worked well enough in my own car as well as the rental cars I've used it with. And my new MTK chipset GPS receiver seems to work anywhere anyway.

It didn't cost much though - a few dollars from Hong Kong.