Wordy title I'm installing a bazooka 10" sub in back. Clearly I'm going to need a cap given the size of the accessory battery. Looking for 1 to 2 farads. The thing is I do NOT want the sub powered unless the car is in powered on mode. i.e. when I pull up and turn the car off, I want the sub to turn on even if I still have the stock stereo going. I've prewired the the speaker level inputs from the JBL amp under the passenger seat. But I need a 12 volt wire that is only powered when the car is in powered on mode as a sense wire to the sub. Another alternative would be a cap that doesn't pass voltage out or charge unless it senses charge voltage, i.e. 13.5+ volts vs 12.5 volts for a fully charged but not currently charging battery. Also, a lot of the caps have voltage displays which obviously can't be powered on when the car is off or it will drain the battery, so the cap either needs to also have a sense input or needs to deactivate itself if it doesn't sense charging level voltage. I'd really like to avoid running a wire from under the front dash if possible but will if necessary. It would be cool if the amp had a 12 volt wire that only get's power when in powered on mode or if there's another wire in the back. There's a harness with lots of wire's coming off the 12 volt battery. Perhaps one of them?