Hi Joshua,
Another Kiwi with an Uno :wave: (note to the rest of the world: the '89 Uno 70 is really an '88 - it's a Mk1)
Most common cause of the locking problem on the Uno is that you have a broken wire going into the door at the hinge area. Take the door panel off, pull the wiring through, and inspect for breaks.
Another cause is that the motor has seized due to the alloy gear teeth having worn out. This is easily detected because it makes the lock knob 'rough' to push up and down.
Have you tried locking your Uno from the passenger's side door? If it locks or unlocks successfully, that means the motor on the passenger's side - or the wires to it - are faulty. If it still doesn't lock or unlock, that means both the driver's and passenger's sides are faulty.
I see that Dick Smith have discontinued their central locking kit, which is a nuisance because the Jaycar (or Supercheap) kits have a slightly different lock motor that won't quite interchange with a standard type. The 'different' type has a pivotting white top.
Anyway, you might also try Repco or an auto-electrician to get a lock motor separately - good luck!
-Alex