I don't know if it will damage your speakers through de-polarization or some other confounded electronic means, but I do know that the sound quality is reduced when wired backwards. (either the bass lacks depth or the highs lack height I forget which one happens)
I just thought of this...since it sounds like the speakers are harder to get to on an SD than on my D, what about the wiring behind the radio?....if its easier to get to, reverse the wires there, and save some time and aspirin.
Good luck!

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'83 300D