VHF radio speakers aren't great, so a louder speaker is a good idea. This speaker can work as an FM speaker by connecting the white wire to speaker + and the black wire to speaker. The yellow wire is for signaling the siren so it can talk to the siren's location. The yellow wire connects to the black wire via a button on the front, but if you're just using it as a speaker just cut it off. There is no 3.5mm plug, just 3 bare ends of cable.
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