IT is not just prestige cars such as Mercedes-Benz saloons that are being stolen in Hongkong. Many cars of little value are taken by joy-riders. The thieves, usually young people, have their fun and later abandon the car, usually leaving it vandalised. It may not be worthwhile fitting a cheap car with an expensive alarm, but a good quality mechanical locking device is a worthwhile investment - especially if the lock is relatively inexpensive. The ''Autolock'', winner of the British National Automotive Trade Awards and the best selling mechanical auto-locking device in Britain, looks something of a bargain at $350 from the Hongkong Automobile Association. The idea behind the Autolock is simple. It consists of a hinged metal rod with a sleeve at one end and a loop at the other. The sleeve fits over the handbrake lever and the loop fits over the gear shift lever (automatic or manual) as a collar. When the Autolock is adjusted and locked in place, it prevents the handbrake being released or the gearshift operated. The device cannot be used for vehicles with foot-operated parking brakes but most cars can be accommodated. The brass lock of the device is claimed to be tamper-proof and features 5,000 different key combinations. One of the biggest advantages of the Autolock over its competitors is that it is lightweight and small when folded. It easily fits inside most car glove compartments and its fuss-free operation means that it is likely to be used by even the laziest car owners. But beware of cheap imitations. There are already at least two Taiwanese copies available in Hongkong. Neither is built to the same standard as the Autolock.