I am no pro but here is my 2 cents...
behind the radio, there is 1 wire with a metal clip that plug into your original Becker system. This wire is an alarm wire which when unpluged (if the car is arm) the alarm will go-off. It looks for a constand "ground" signal and when the signal is broken off, i.e. unpluged, the alarm will go off.
Just re-connect that wire to some ground source. However I know that in the early models, if the wire is getting no ground before the alarm was arm, the alarm system is smart enough to ignore that wire. Anyway, just try connecting that wire back to some ground source first.