I think you can get a module from a higher spec Fiesta or Focus and it should be plug and play. On the Focus it lives above the accelerator pedal. Google Focus Remote Locking and there is quite a bit of info on the different modules and the ones you actually want,

I thought about doing it to a Sierra but have just bought an aftermarket kit because it seemed easier and they were able to supply a blank key-blade to suit the Ford locks. Still to fit it, mind you!