99 Silverado Wireless Which one??

I have a 99 Silverado LS ext. Cab that has a wireless remote for the door locks, and supposedly remote start. How can I determine which
remote I need, the truck didn't come with one. The idiots at the place I bought it tossed it thinking that it belonged to a truck they had sold a long time before I bought mine.
Anyway, I know it has remote entry; the remote start may be aftermarket. It has the factory GM alarm system that honks the horn and flashes the lights if the doors are unlocked from the inside ...like breaking a window and reach in and unlock the door.
Any help would be greatly appreciated!
Thanks Ed
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