Ok i acquired a pioneer deh-p6700mp tonight for free. I noticed on the back of it that it has a 3.5mm jack(atleast it looks like one) Its not labeled or anything. I can plug my mp3 player into it and making sure aux is turned on i dont hear anything. now if i turn the vol. up wide open on the H/U and the MP3 player i can just barely hear it. Do i need something else?