so I recently bought a z31 for 3k, and I just put in a working battery and the interior lights turned on, the headlights worked, but the security light was flashing, and when I put in the clutch and tried to crank it over it like blew like a fuse and just disconnected itself from the battery, then I reconnected it and opened the driver door and the alarm went off, can anyone tell me why this is happening or how to fix it, btw the key is not a stock key idk what happened to the stock one

 

Under the passenger seat is an alarm box, Just disconnect it and the alarms will stop