Bought this boat used. Have had it for two years. Every fifteen minutes or so there was an interference in the stereo system. Loud and annoying, amp, lower speakers, upper speakers, then radio. While pulling the radio out I found what looks to be a gps unit (small black plastic, round on one end, square on other). Opened it up had a sim card in it, like a cell phone. Had green corrosion in it. That was my radio problem. Wondering if this was a factory option or aftermarket install?