Currently, cars are identified via three pieces of ID.
The license plate is given to the vehicle at the point of registration but the plate is easily replaced and therefore provides no secure ID for the car.
The engine is given a ID plate as is the chassis, but these a little more than a metal
sheet rivetted to the chassis itself. Harder to replace certainly, but not impossible. Criminals looking to "ring" a car - ie. give it a new ID, would have no problems.
I propose a ROM chip for the car that stores the engine and chassis numbers. At the point of registration, the license number is programmed in, but is then fixed. The chip should be wired in such that tampering with it would destroy important engine management circuitry and thus make the car unuseable.
I would further propose that the chip can be read with a handheld by the police - much like a pet chip for example. That way, the cars ID can be identified without getting into the car.
If we as drivers were asked to register our roadtax against chassis number, rather than license plate number, the handheld could then be used to check for road tax expiration.