A relatively recent trend is to use GPS systems in connection with car-mounted cellular telephones to automatically locate cars involved in accidents. In this system, a microcomputer monitors the airbag deployment system installed in new cars. If it detects that the airbag has deployed, the computer calls a service center over the car's cell phone, passing it the last known location of the car as determined by the GPS receiver. The service center passes the information to the local emergency services who can then respond. I'll have more information on this as I find it.
Many companies maintain large fleets of vehicles.
Two of the problems associated with maintaining such a fleet is knowing the current location of any given vehicle and knowing it's physical condition. Fleet tracking systems are concerned with addressing these two issues. Other systems, such as Loran may be used. Systems such as Qualcomm's Omnitrax service use proprietary satellite-based location systems. A vehicle mounted communications device The vehicle being tracked must be able to transmit it's location back to the fleet control center by some means.