1975-1980 Dodge AM/FM Radio w/ Cassette player

[flcl64]

I just got a radio for my truck and it works alright, but I think the cassette player is kind of separate from the radio. I don't think they run off the same power wire. There is an extra yellow/black wire with a redish brown plug that I have no idea what it goes to. Does anyone know if that is the power to the cassette?

I tried again today and I heard a motor running, so it's getting power, probably needs a belt. I think the yellow/black wire is a trigger wire for an automatic antenna based on a manual I found online for similar radios.