Factory Radio Wiring

So a friend picked from a warehouse that had hundreds of radios and found this Model 228 for my Barracuda for fairly cheap. It's the right model I think for the Valiant '65/'66. Either way it's the right fit for what would have come with my car. I cleaned up the inside (blew it out), lubricated the moving parts and bench tested. Everything seems to work ok, not 100% on the dial light, but the bulb work on the bench. Not sure about the socket.

Question is, the factory wiring seems intact and I know what all the wires are, but not sure where the ground would plug into as someone before me removed the original speaker (I had a unit from a '63 with its speaker to use for bench testing). The black ground wire had a plug with a male spade, so was there a pigtail that was connected to the speaker with the female or does the head unit not ground to the speaker?
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