A work mate's partner started up a company that wires in a gps tracking system that not only tracks the vehicle but disables it any where there is cell coverage.
They do this for a car finance company. If someone stops paying the bill the car is disabled until the funds are paid. Once they finish paying the car off they have the choice of buying the kit as an alarm system.
I've asked her to give me a price on the system and installing it.
Watch this space.

A few years back now Nissan Skylines were going missing buy the dozen per week. Their system was what the police used to foil the shits doing it. Since then they've been a successful privately own local company.