The tamper circuit is a closed loop from control board to door switches,pir's and bell box, plus one built into contol panel box itself.
The system monitors for a break in the loop. It is there so that if wires are cut,the alarm will sound.
There should be a fuse on the circuit board,from the transformer,check it is ok.
Disconnect battery terminals and check with your meter that you have 12v at the wires.
Its possible that the battery has an internal short circuit, if you have never had it replaced in the 6 years you have owned the property, the battery will almost certainly be past it's use by date.
Pete
February 2007