What I'd like to do is take a simple mute signal from a cell phone (change to 12V from 0V) and have my stock head unit see only 12V for a few seconds. I want to put a resistor inline and mimic a "mute" button push from the steering wheel. I'd like a similar pulse of 12V when the phone is hung up and the voltage returns to OV. What I'm trying to avoid is a constant 12v signal to the headunit when this circuit is set up to see 12V for short periods (button pushes on the steering wheel). Can anyone help?