I have the stock radio so that's not the problem. Hence, I removed the wire to the alarm horn this afternoon. More like a 3 min job, no tools required. Just open the front trunk, remove the drivers side cowl cover, press the top and bottom retaining springs on the connector together and wiggle the connector off. Cover both sides of the connection with electical tape, then zip tie the cable/connector to the bracket so its not flopping around. Replace cover, close trunk. Done. And now, no more beeping when I lock the car!
Here is the location:
