radio wireing

im trying to hook up my sony receiver in my 735i and i cant figure out what wires are what from the dash...i found out that the white is the antenea because when i hook up purple stripe to ground and white to antenea, the antenea moves, but i cant figure out what positive is because when i hook up positive to purple, it doesnt turn on the receiver. if anyone knows what wires are what...please let me know...