I did something like that for my deck. I got a basic small amp, sealed it in a rubbermaid tub and hid it inside my BBQ cabinet. I ran a line in jack (3.5mm headphone) into it. I have just a 3.5mm wire, and switch inside a waterproof box.
So anytime you are on the deck, you can plug in your phone and have instant music, and just use the volume on the phone to control things.