3 wire alarm problem noobie

Hi I have just fitted a simple 3 wire alarm (compact 1000) to a nissan micra 1999. I wired it to the constant positive for the cd\radio which I was fitting at the same time The alarm works fine, the shock sensor works and all the setup beeps are right but the alarm does not go off when you open a door ie the current sensor. I have fitted these before but wired them to the fuse box, could the fact that I have taken the live from the radio make any difference. any other suggestions gratefully received.
Tony