Dean - You replaced the Access 2 module (Not 1 or 3)? Good to know that did not solve it.

Swoop - I will give that a shot. I did check it with a meter and saw pulsing voltage, so I don't have high hopes. What do you mean by a "manager card"?

Some more information I found when I was there yesterday...
CA404 Pin 6 on the Access 2 module is the Travel Alarm Output and I am seeing a pulsing voltage on this pin. I suspect this is why the volume is so low. If it was solid voltage, it would be louder.
When CA405 is unplugged, the alarm continues. As soon as I unplug CA404, the alarm shuts off. So this should mean that the problem is not in the CA405 harness. Time to look more at the CA404 harness. 12 of them are outputs, so that leaves me 7 more pins to check....