DC2 Integra non-remote central locking

[YellowLine]

Hey all

Just joined OzHonda after purchasing my first car - a 1998 Integra GSi. Am looking to fix a few little issues it has - the first is the central locking (key, not remote) is largely inoperative and very glitchy. In short, opening and locking the driver's door does NOT unlock the passenger door. However if you fiddle with the lock on the inside of the car (not necessarily switching it from lock to unlock, but merely pressing it in certain ways) it does seem to lock/unlock the passenger's door.

Any ideas about what might be the problem, and any possible DIY solutions would be much appreciated