water temp gauge

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I want to change my existing gause which is just red and green to one that read degrees. The boat iss a 1976 Sea Ray and needless to say behind the dash looks like a rats nest. Any way I bought another gauge and couldnt get it to work because it would only work with their sender. Now I am confused and just trying to get the old back to working and cant seem to get that to work now either and along the way my amp gauge has stopped also. I can see where a couple of the wires go as far as from the key. The gauge has an S and it looks like an I on it. Any help would be deeply appreciated as it seems I am lost. Thanks So Much Jerry