You still have drive-ins near you?
You actually WATCH the movies at the drive-in?
Errrm,,,
I'm not sure what makes that circuit work - running a different power wire to the radio might work, but if the circuit is internal to the radio, it might not.
Will it still shut off after 15 minutes if you leave the key in the ignition in ACC mode? (Ermmn - okay - no key, but if you press the button to the ACC position).