David, the interior lights are driven from the door by a switch inside the door itself (not a switch on the door jamb as is the case with many other cars). To get at it, you’ll need to remove the door card, which is not hard but
is rather long-winded - many screws. Lots of posts on here describing how it’s done.
The central locking
on the S2 (SE and NA are different) on my UK car is driven by the alarm system. However, I note that you’re in Australia, and I’m not sure whether ROW cars had the same alarm, so treat this information with caution. Setting the alarm sends a ‘lock’ pulse to the door lock mechanism, and vice versa when it’s unlocked. On a UK car, I’d be checking for electrical continuity in that circuit. What kind of alarm system (if any) does your car have?
It sounds to me as though the two problems could be related, and I suspect a previous owner may have “improved” the electrics

Has this issue developed recently, or have you always had it?
By the way, manuals and circuit diagrams are always available under the ‘
Manuals’ tab on the menu at the top of this page