I don't even know how the locking wire works as I never studied that part.

I can only suggest to start from studying wiring diagrams in ELSA. If you don't have it please install it. I will never wire anything without ELSA diagram.

Central locking ECU I think should be a part of central locking pump assembly in the boot on the left hand side. But I don't know it for a fact because I never removed trim from that side of the boot.

My project was different. I just located the central locking wire near driver's door and found a way to attached a second button to that wire.

thanks for your help. i have ELSA, i just need to reload it as my pc crashed.
will give it a try

I would suggest to study diagram for your car first, i.e. how lock wires are connected and how they work (ground signals?). Then study diagram from a later car with the central locking button and compare.

If the only difference is additional ECU wire and the rest of pin out is the same then I'd get a pump from a newer pump and snake that wire from boot to driver door.