As we don't have a clue what alarm it is, it's difficult to help, and an '89 has a very different electrical system to the '93 onwards XJs we have in the UK. As far as the wires arer concerned, the black one is probably the antenna for the remotes, some may not be used as it won't be a vehicle specific system. First thing to do is make sure you have a decent, fully charged battery fitted. If it still plays up, best thing to do is remove it. Trace the wiring and find where it is spliced into the vehicles's original harness. Where you find a wire cut and rerouted through the alarm through a pair of wires, for example the starter circuit, remove them and rejoin the wire, single wires can just be removed.