If it's a DEI alarm viper, python, sidewinder, hornet etc press button 1 then 2 then 1 again with the door open. The lED will go solid then. If it's a newer DEI alarm with the lock and unlock symbols try lock, unlock, lock with the door open.
If it's another brand pm me and I'll get back to you. I worked as a 12 volt audio/alarm/convenience installer from 96-00 before finishing school and moving into composites and Fiberglass prototyping.
We also have a retail store that does car audio/alarms/remote starts etc so I have access to pretty much any info I need.