Golf IV Remote Locking not working?

I removed the stock Gamma Stereo, replaced it with another Alpine unit, now the car won't lock or unlock using the remote Keys. The Central locking still works using the key,having to turn the ignition to stop the alarm etc. Do I have to reset the remote part of the key? I tried following the manual where it says about synchronising the key etc. but to no avail.
Help!!!
Thanks in advance