My alarm goes off by itself?

Hi,
It seems you now have two problems; first is the alarm, and next the ECU light flashing after you disconnected those plugs. The best way to sort your alarm is to try and identify the manufacturer, it's not always a generic "vauxhall" system. Once you have the make and if possible, the model of alarm, you can search the net. The makers of alarms are really good at supplying diagrams, info, etc, unlike the car makers themselves. The ECU is a different issue, the best source of info is the vauxhall owners web site, where the knowledge of specific vauxhall snags is very good. It sounds like you might have pulled and reconnected a plug that feeds information to the ECU, and apart from disconnecting your battery for 20 minutes or so to "reset" the ECU, I'm not sure what to advise.
Cheers now

Ken Chapman
June 2007