We did both
I have a 15inch sub and all updated amplified inside speakers. I kept mine on battery the whole time. I started between the movies. My battery was fine.
I kept my car running. It's better to bring an AM/FM radio to listen to. There is no app that I found to download to listen to thru a wireless speaker. But my car has a nice sound system so I chose to waste my gas (it was worth it)
We keep are car off with the radio on during the movie, then turn the car on during credits till the next movie starts.
No just remember to turn your car on every so often.
We did both as well but most people just leave the radio on. I have seen plenty of cars needing a jump afterwards.
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