Have you actually checked to make sure there are speakers hooked up to it? Someone may have just thrown in a head unit and attached power to sell it.
EDIT - Most factory boat stereos only have 2… less then desirable speakers anyway. Both may be unhooked at the head unit or at the speakers themselves. You can use a simple 9 volt battery to “POP” the speakers and find out if they are functional and which one is which. DO NOT hold the battery on the wires, just briefly touch them and see if they react. Some lower power head units use a common ground for all the speaker wires… this screws a lot of people up. Don’t count the unit out until you have done some more trouble shooting.
Kind of love boat stereos..
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