2006 focus remote sensor ?? HELP !!!!!!?

I have a 2006 ford focus estate the keys/transponders are programmed to open/start/drive the car manually but I cannot get the fobs to open the car, I have even tried a brand new fob and had the others tested - All ok, so I can only assume its the remote sensor reciever but not sure 1. How to Test 2. Where it is 3. How to change it.
Please some1 HELP !!!
Ford are useless !