LS1Porch

I picked up a new car that has a factory alarm (1990 S2). I did some searching on it here, but it seems like most of the responses are "Don't touch it" or "Bypass it" (which i found instructions for).

My question is--what does it do? It doesn't seem to have a siren anywhere, so i assume it just kills the starter. Mine also doesn't have the weird 3rd lock cylinder. How does it know when to disable? Was there a keyfob i didn't get?

I've got a decent aftermarket alarm that i'd like to install in its place. Seems like it would be pretty easy to just swap one out for the other. Does anybody have the wiring diagram for the alarm? It's not even mentioned in my shop manual set.