I was able to get my Catalyst and radio working together pretty easily. From Amazon I bought a Y connector (mono) and an extension cable (mono, 8 foot?) and just plugged both sources into the Y and then into my ear bud/speakers. I have yet to have it on the track but it works in my living room. Let me look at my set up tomorrow and I can tell you what I purchased. Some folks are using bluetooth but I have heard of volume issues. I also considered running both helmet speakers and ear buds together.
Now that I have thought about what I am doing. Is it possible that I am backfeeding radio transmissions into the Catalyst and causing damage?