Does anyone know if it's possible to put a back-up battery siren on the stock alarm? It looks like the DEI 515R is universal enough to do something like this. It's only 3 wires, power, ground, and siren trigger. Seems like it would be a good back-up device to the stock horn. Or would something like the 520T be better where it's just a battery backup?

 

The 515R is just a powered siren essentially.

If the alarm goes off it will trigger the siren (probably on way to do this is from the 12VDC horn wires under the hood).

If the power lead is disconnected the siren will go off until the reset key is inserted and turned or the internal battery goes dead.

Not sure how you would go about using the DEI battery backup with the OEM Honda alarm. You would need to insert it in the 12V power source going to the OEM alarm. However, since the OEM alarm uses the cars horns it would not do much for you since the horns would not work if the battery was disconnected.