Spectra A9 External Radio Input?

[KI4IHC]

I'd like to hook up another radio to my spectra siren. I know in the RSS there is a External Radio B (I think that's what it's called) that I assume routes External Radio B through the siren, but I haven't been able to find any more information on it. Can I hook it up via the VIP inputs, or is there somewhere else that I can connect it? (Or is there no way to hook up a secondary radio to the siren?)

The radio is a Spectra A9 VHF, and I have an Astro Spectra siren.



Also - I was torn about posting this here or in the Motorola information forum, but finally decided on here because it has to do with sirens, and such.