You'll have to climb into the boot and remove the black rectangular piece of plastic from the inside of the tailgate. This will give you access to the lock and catch release mechanism, it should be easy enough to work out what to push, pull or press in order to open it.
Rear locks are quite prone to seizing up through lack of use on a mk2 fitted with central locking (due to the fact that the shape of the rear end sucks so much grit and crap up from the road surface).
