How do I fix a Audi A4 central locking pump?

I have got a problem with my central locking on an Audi A4 1.9 TDI 1999. It does not work with the remote or when I try to open the locks with the key.

I have to open/lock each door individually by hand, apart from the drivers door which I can open with the key.

I've had it checked and have been told it is the central locking pump. It controls the central locking, electric windows and interior light I think.

Every thing else is working apart from the central locking as I have mentioned.

What I want to know is that is it the pump.
if not, then what is it.
if yes, Can the pump be fixed. If not where can I get a replacement from.


Thank you