The entire remote unlocking control is housed in the central locking module (the pump), which is why replacing it may have been suggested in the first place.
There is a repair procedure for the receiver aerial, which can cause reception issues with the remote.
Can't find the procedure currently, might be on Audipages, or maybe I found it in the tech dump?
There is an aerial hack used for B5's
here.
For reference, the central locking module on the D2 is on the left hand side of the boot above the left hand wheel arch (requires a fair bit of trim removal).