Does it go off at random or when you open the doors?
Is the car battery in good condition? If it's a bit low on power this can trip the alarm.
Has the car been giving a series of little 'beeps' when you turn off the engine? If so it's probably the power sounder battery failing, though I wouldn't expect this in a 2004 car
Next most common cause of random alarm is broken or badly adjusted switch under the bonnet.
Make sure the bonnet and all doors are fully closed.
ptd, June 2009
