I purchased my car back in 98 and got the local dealers to fit a replacement alarm system. It has been working with no problem until recently when if left for a couple of days would go off and also if you disconnect the main battery the alarms backup battery doesn't kick in.
Is it easy to open the horn enclosure in the engine bay to replace this battery? It has two Philips screws on the back of the unit next to the keyswitch which I have removed but it doesn't seem to release anything and can't see any way of removing the unit off it's fixing bracket?
Would the backup battery cause the alarm to sound?

Thank you all for any advise you can supply
Kind Regards