car alarm

hi ya i dont know if theres anyone out there that knows. i have a gemel GR99T alarm on my 1991 mk1 golf convertable. i armed the car when came back wouldnt diss arm. so had to take siron bit off to get me home car still strated after doing this. thought it was just battery in fob as it wasnt lighting up when press the arm butten.. got new battery put siron bit back on but still wont dis or arm the car and siron just goes off all the time so have had to take it (siron bit) off again till find out whats wrong???? any ideas??????