The book says it's the flap under the light switch, but this isn't the case!! Is it the big side panel at the end of the drivers side dash? When u open the door?
 
Took my car in today to central seat, they are replacing the volumetric sensor, 2x new mirror mechanism's too, should be all the problems solved now, cool
 
Well I locked the wife in the car today by accident when I popped into the post-office and found out 10mins later that the alarm works. She was not amused...