How can i install a stereo on my new yamaha snowmobile?

I allready have an fm transmitter that plugs into my snowmobile lighter plugin that transmits my mp3 files from usb stick to an fm station and wireless headphones, But they dont fit in my helmet and the stereo frezes up. How can i install a normal mp3 player or something that will plug into a lighter output and put headphones on it or a speaker in the back so ill have sound? Its a quiet new 2008 sled 4 cylender so it doesn't have to be too loud, and remember i have an auxilery port on snowmobile for plugging things in and i can also wire anything into the 2 batteries. Thanks