'74 Duster Radio

Gotta question, I knew this stuff 38 years ago when I was doing this stuff all the time, but age caught up and I just don't remember anymore lol. My dash speaker is not working, so I want to wire a pair of speakers I put up in the back window to the factory am/fm radio. I know it won't be stereo, just two speakers instead of of one. There's a green and a black coming out of the radio. The speakers have two leads, the green and green/black and the grey and grey/black, (left and right). What goes where? Also, I want to wire my old under the dash cassette player in there too. It's got four wires, a red power, black ground, and a green and a grey. Can I wire this with it going to the same speakers along with the factory radio? Any help appreciated. Everything I find online is modern harnesses...