central locking mod

Hi , Can somebody explain how the central locking works please . I wish to operate central locking with key only . Its a 97 disco 300tdi
I removed drivers door card and found no voltages on any of the 4 wires , but 12volts on two wires when the key is turned to open or close doors . Is it a pulse or what that supplies commands ?
Is there a 12volt aftermarket master actuator I can buy to do the job to control other slave doors without changing all the units.
I wish to turn off alarm unit by removing fuse1 on satellite 1 . All other functions have been bypassed i.e . starter ,diesel pump etc .

Yes there is, u will need a new master actuator plus the ECU usually comes with a remote fob which is part of the kit, which U don't have to use, see eBay or Maplin for the kits which come with a wiring diagram.
The existing two wire slaves can be reused.

Thanks