Alarm Car Won’t Start

2000 Mustang alarm will not reset, car won’t start, system thinks it’s being stolen.?
When I purchased car it did not have a alarm remote. So far I have tried waiting it out by disconnecting the battery (with no luck), and tried using the key on every door and the trunk to disarm system…no luck. Every time I put the key into the ignition the car goes into alarm. HELP!!!
i had got an 97 stang and same thing happend to me there turnd out to be an toggle switch under the dash on drivers side have you tried looking for one?
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