*reopened*
The question asked was refering to using a DEI 515r Siren (with back up)
LINK with a Viper 791xv Alarm
An installer told the OP it couldnt be used, the correct answer is:
Yes, you can slice the DEI 515R backup battery siren into the siren output wire from your already installed Viper Alarm.
You attach the RED to + 12 volts power
BLACK to chassis ground.
Probably you can push all of these wires from the 515R under the hood through the same hole (usually a grommet) in the fire wall that the siren wires run through now.
Then you can make this red 12 volt and black chassis ground wire connections under the dash.
Then you splice the VIOLET (+) siren input wire to the (+) Brown siren output wire from your Viper. The brown wire is in the 12 pin white plug in your Viper CPU. You will then have two sirens, one standard, and one with a back up battery. Its no problem to have both........
GREEN wire is NOT used