You just purchased a 1998 Passat it came with no manual you cant get the trunk to lock is there a secret you should know about?
The trunk automatically locks when the central locking is
activated. Do you mean that the trunk remains unlocked even when
the car doors are locked? The lock systems on these cars are not
good either in terms of logic or manufacture. You may find this
link, which details the repair to a door lock, helpful.