I haven't used it, but I've seen the wiring. That auxillary plug in is connected to a female to female auxiliary plug, and the ausillaries from the radio plug into this. The connection is under the switch panel. Unscrew 6 screws and lift and you will find this connection right under the plug in. It sounds like one of your connectors are grounding out, or not plugged in at all.
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Boat Name: King Kong
"Boat + Water = Fun"