I am not specifically sure of what you mean by the "common rail" as on a diesel engine the common rail is the high pressure fuel rail supplying the injectors. did you mean this or did you mean the Canbus system which is like a computer network all around the van?
If installed correctly, then it is likely that the alarm is taped into several things all around the van, door locks, indicators, fuel pump, ignition etc, and the wiring for these alarms is all black so there is no colour codes to be able to chase back what they are connected to there are no pin outs from the alarm ECU to be able to reference.
I would leave it for someone either experienced with vehicle electronics or call in a professional.