Changing a siren to horn out put.

I put in a magicar alarm keyless entry system. I want to change from the siren that the system came with to my horn instead. I don't know what kind of voltage the magicar system puts out to the siren whether it be plus or minus. Does anyone knows what kind of relay  I need and how do I hook it up to so my horn will activate instead of a siren?

Thanks, Mike