Vista 21ip Can't Get Bell To Activate Relay

I just replaced my 12 yr old Brinks alarm panel with a new Vista 21ip based system. The conversion was relatively painless and I have everything working except the siren. The Brinks system used 2 small speakers and because of the placement of those (vaulted ceiling which is hard to get to and would require painting/touch up if replaced with something of a different shape) I decided to keep them and use a Moose MPI-11 siren driver. I do not know the specifications of those speakers - they were connected in series to the Brinks panel "siren" terminal.

After hooking up the MPI-11 I tested it against the AUX (terminal 5) port and the siren sounded fine. Connected to the bell output, however, produced nothing during an alarm condition. I assumed the bell output did not have enough power to drive the audio board - so I installed an Altronix RB1224 relay set in 12VDC mode. Here are my connections:

Bell (terminal 3) -> Relay board pos+
GND (terminal 4) -> Relay board neg-
AUX (terminal 5) -> Relay board Common 1
GND (terminal 4) -> MPI-11 ground
Relay board NO 1 -> MPI-11 steady+

If I disconnect the wire from the bell and touch it to AUX (terminal 5) the siren goes off great. However, when connected to bell, the siren still does nothing when it should be sounding. Putting a meter across bell (3) and GND (4) shows 13.5VDC steady when the alarm is sounding and .5VDC when the alarm is off.

Am I doing something wrong? Any ideas on what might be the issue or how to troubleshoot further? I don't have any compatible sirens to try but I guess I could buy one to verify the Vista board alarm actually works at all. Also the entire system is brand new, including the 12V 7AH battery (which tests at >13VDC with a meter for what it's worth).

Thanks in advance for any help. I called Honeywell tech support but apparently they don't like DIYers as they refused to talk to me as an end user. Good times ;)