Probably speaker wires. I noticed the same thing on mine, extra plugs that didn't go to anything. Got to looking around and figured out the two front corner boxes were pre-drilled for speakers for boats that come with more than just two like mine. Since I was installing a different system anyways I bought two more speakers and simply cut away the vinyl covering the holes and connected a 12" splice of wire to the terminated plugs. Easy upgrade to four speaker sound.
While doing that I also discovered more wires that are connected to an unused rocker switch on the dash. Turns out the front corner boxes are also ready for additional task lights. Same thing, all I would need to do is buy some lights and run a short splice. I didn't bother since night boating isn't an option here,
