I had the same problem in my truck two years ago, after closer inspection they told me (circuit city
) that my wires were damaged going to the speakers. All in all it wasn't the case it was the wires behind the head unit that had trouble. But this could be the same problem. We know the unit has power, but it isn't getting sound. Sounds like a definite speaker or wire to speaker issue. Does it have waterproof speakers installed? the wires could be frayed, check them out before professional help, it ended up costing me 60 bucks for a simple problem.