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Think I would look at the instruction sheet that came with the echo unit.
Usually, the mic in wire on the echo unit goes to the Mic pin on the mic plug. The mic out wire on the echo unit usually goes to the radio board where the mic wire from the plug would have gone.
Usually; but not always.
Other than that, the red power wire goes to either the +8V regulator output or the back of the radio's on/off switch for +13v.
But I wouldn't do ONE SINGLE THING until you identify exactly what echo unit it is, and find the EXACT installation instructions for that particular model. Probably find it online if you can identify that board.
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