As to fixing your tailgate lock...On my '98 Laguna RT Sport estate i found that a pin was broken in the lock which stopped the central locking working..You will have to remove the tailgate liner to check this out, and strip the lock mechanism. I fixed mine by using a countersunk headed screw with a long shank (of the right diameter) cut down to suit. Fix this first and it MAY cure the alarm going off.
Ray Murphy
April 2006