The box would be tied into the column down near the floor. You should see 3-5 yellow plugs spliced into the factory wiring
On the after-market alarms I have good luck unplugging the 2 or 3 plugs in the harness and leaving them that way, those are the trigger wires. The larger wires, usually red, purpish, yellow, or orange you can just leave alone and plugged in.
Try unplugging the small wires from the factory wires. Just make 'em in case you have to put 'em back.
they're usually behind the knee bolster:
After-market alarms usually have connectors like this. The tiny wires are the trigger wires:
By unplugging them you're disconnecting the alarm but the original wiring is still in place and should work as if there was never an alarm