How to drive a relay from audio?

I've bought a replacement car alarm but just realised it has its own siren driver, and no steady 12v "alarm is running" output signal, which I need for the 2 in-car piezo "screamers" and external pager that the old system used.

Is there a simple way to use the siren output to drive a relay?

If the siren driver has an AC output then rectify and filter it into DC and use it to drive a power transistor instead of a relay.

You could just replace the piezos inside the car with another siren.