When you go from the RUN to ACC position and then from the ACC to the CRANK/RUN positions, there is an interruption in the switched 12V turn-on signal to the head unit. So it shuts off and back on. This can wreak havoc with blue tooth. Sometimes its a short pause in the music, sometimes, you have to re-pair the phone.
Replacing the ignition key switch with a manual rocker switch keeps the head unit turned on, regardless of the position of the key.