second siren on viper alarm

How do I add second siren to car alarm?  I heard I may need a relay? 

you can use a positive to positive relay,

87 to 12 volt, and fuse it with a 5 amp

85 to ground

86 is your input from your alarm and

30 will be the output to both sirens.

In the following diagram the wire labeled remote turn on lead is the siren output wire that comes from your alarm.  The wires labeled fan are where you connect your sirens.

Connecting Additional Devices to the Remote Turn On Wire ( link)

Using a 30 amp SPDT relay, connect terminal #87 to constant 12 volts positive with a fuse rated to the sum of the additional accessories you've added and the components you need to turn on. (If you have two fans rated at 5 amps each and a neon light rated at 10 amps, you would use a 20 amp fuse plus 200 ma for each amplifier and processor.) Connect terminal #85 to ground, terminal #86 to the remote turn on wire from the head unit, and terminal #30 to each accessory with an appropriate fuse. A fuse (not shown) could also be used between the output of the relay (#30) and the remote turn on wire of the amplifiers and/or processors for extra precaution.

Thanks sounds pretty easy.