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You need to find out what brand/model the alarm is, get up under the steering column with a flashlight and follow the wires that don't look stock.
Remotes aren't universal so you'll need to find a dealer for that brand, but all alarms made in the past 15 years are easily programmed to the system, make sure when they program them to wipe out the old remotes just in case......
Any reputable alarm/car audio dealer will help you.......if the unit is not older than 5-6 years old it should not be a big deal.....