Why is music playing through my bluetooth when I want it to play to the audio jack?

I have my iPhone playing music through the audio jack directly into my stereo, but when I pair my blutooth, it starts play through my earpiece instead. Sometimes it does this, sometimes not. Cannot figure out why!(?) When I have my audio jack plugged in, I always want it to play my music through that output. But I want my blutooth paired in case I get a phone call. I need the two to be seperate.