It's certainly possible you have an aftermarket unit. However, if the theft light blinks on the dash, and lights up when you lock the doors and are getting out, then it's probably a factory unit. Also, if the alarm sounds, and you start the engine and it stops, that's a factory unit.
Here's where I'd start: Disconnect the RAP module (inside the jack compartment. See my gallery link in my sig for more pics), then find the pin connected to the DG/VT (Dark Green/Violet) wire. Then, connect a multimeter to the pin set up to check for continuity to ground. Then, have someone turn the key in the door tumbler to the unlock position and check to see if you get a connection to ground. Check all three doors this way. It's unlikely that all three will fail to ground the connection. If all three do, then you have a bad wire somewhere between the connector at the RAP module and the splice where all three come together. That may mean pulling out some interior trim to find the splice (I have no idea where it would be).
If all three switches ground properly, it's possible that the RAP module itself is bad. If sending a ground signal doesn't tell the RAP module to cancel the alarm, I'd think real hard about it being bad. If you could swap it with a known good one out of a friend's car, that would tell you for sure....
-Joe