It sounds like you have the wires from the speakers connected to the right side outputs. Double check the colors, and make sure you have one pair connected to the right side output, and the other to the left.

With Sony (and most other car stereo manufacturers) you have four speaker outputs:

White == Left Front +
White/Black == Left Front -
Gray == Right Front +
Gray/Black == Right Front -
Green == Left Rear +
Green/Black == Left Rear -
Purple == Right Rear +
Purple/Black == Right Rear -

You need to figure out what wires go to the left side speakers and what wires go to the right, then connect the left pair to the green and green/black wires, and the right pair to the purple and purple/black pair.

To help figure out which wires go to which speaker, get a 9V battery (one of the little ones) and try lightly touching the speaker wires to the terminals. The little batteries can't put out enough current to do any damage, but you will hear scratching noises from the speaker once you've made contact.

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1984 300TD