no start after cutting siren wire

Hi ... trying to help out a friend. She has a 86 Mustang with a security system installed the car worked fine then she decide to cut the siren wire off, now the car never started again. I don't have any experience in security systems but I'm sure someone can tell me that that's probably why now her car won't start. Any help would be appreciated on how to reprogram or whatever needs to be done. I will find out the type of system she has.Thanks.

They probably installed Starter kill on the alarm. This will keep the car from starting when the alarm is armed. When she cut the siren wire, this just killed the siren, but the alarm is still armed. Follow that wire from the siren through the firewall and it will go to the alarm brain. Try unplugging the larger harness from the alarm brain. It should be the same harness that the siren wire is connected. If the car will still not start, there will be a wire that goes to a relay that is probably located very near the harness behind the ignition switch. There will be 2 larger wires on that relay. Follow the 2 larger wires and they should both go to the same color wire that was cut. Reconnect the 2 ends of that wire and try it again.