If you are trying to stay on a budget and just get a stereo, your choices are limited. You are better off with one of the xm/sirius portable satelite recievers, an antenna on your bike, and a bluetooth system for transmitting it to a bluetooth audio reciever in your helmet.
However, some GPSs come with either satelite radio functionality, or the ability to store and play mp3s or both and all you do is add the antenna for satelite. They are designed with built in bluetooth to transmit directions, and music. You spend more this way, but you get more functionality, the wiring is easier and you don't have to cobble together anything.
There are plenty of people here who use bluetooth helmet audio and will be happy to share their recommendations with you.
Dan
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I ride on two wheels cause I feel very unsafe on one wheel.Dan in Miami. The king of custom! Check out my bike mods
hereReality is irrelevant. Its perception that counts.
Control people's perceptions, and you control their reality.