Central locking

Hi everyone, I hope I have the right section this time. I have just bought an 96 A8, I was after something as quick as my old scooby and as comfy as the old lexus so I hope ive got the right car this time around, (no 53) anyway to the point, the main problem I have is the central locking, it all works with the keys but not the fobs (checked batteries) does anyone have an idea what the most likely cause would be? I have checked the fuses and they are ok. What is the central locking pump and where can I find it? could this be the cause?