Hi Jason!
The easiest way is not to fit the FIAT parts, but just to get a four-door central locking kit and install that.
If you instead fit original FIAT parts taken from another Uno, there will be quite a lot of work swapping latches etc. and there's no guarantee that the lock motors are still working or will continue to work... plus the wiring loom, fuses etc. may pose a challenge to hook up.
Once you have removed the door trim panel, you'll see a lever that's connected to the door lock pin. It has a spare hole for a central locking motor (or some other purpose), so all you need to do is watch how the lever moves in an arc, and then mount a central locking motor so the moving part operates in a straight line. This means that the motor ends up mounted on a slight angle for the front doors. You'll need a 3mm drill bit, and some patience, since one of the two holes required will probably end up on an inward curve in the door metalwork (easy to break drills; ideally centrepunch first). Put one screw in and then drill the hole for the other screw. For the back doors, you don't have to drill any holes: they're already provided for standard central lock motors!
The central locking kit comes with metal rods for connecting the motors with the lock levers. You'll need a hacksaw to cut the rods down to length (cut halfway, then just bend and snap). The kit also comes with 'joiner' blocks to attach the rods onto existing rods in the door. These aren't needed for the front doors but, from memory, I think they are useful for the rear doors which don't seem to have anywhere on the linkage to attach to (ironically, considering the holes provided for the lock motor mounting), so you attach the rod to the lock button rod at the front of the door. Takes a little error-and-trial to get it right, but easy enough.
I like to use corrugated tubing to get the wires into the door. All you need is a round hole in the existing square plastic plugs you find in the door frame (these holes can be made with sharp scissors!) Then fasten the tubing using cable ties. Make the door-pillar-plug hole a tight fit on the tubing, and the door-frame-plug a loose fit to allow the door to open and close without kinking the tubing (and of course, the wires). Don't be afraid to spend ten minutes each door getting this right, as it ensures long-term reliability
A source for the power is the radio/cigarette lighter power supply... a good earth is the heater mounting nuts on the bulkhead. If you want to use the fuse that comes with the kit, run the power wire through a bulkhead grommet (at the top, near the heater air intake) and connect to the junction block on the battery positive cable.
I think it's a good idea to add this feature, as it certainly enhances the convenience (e.g. with letting a passenger in when it's raining, or from not having to lean all over the place to reach the locks)...
The four-door kits are sometimes as low as the equivalent of 15 pounds in our accessory shops; they are strangely common (I mean, you never see kits for electric windows or air conditioning...)
I'll see if I can find some pictures to attach
Cheers,
-Alex
