MACSVIN, Even though am not expert there, I would suggest the following.
Directly connect audio power to battery.
Have a separate switch control for audio.
To me this is the only option I strikes my mind. But this could be risky as if you forgot to switch off the audio [especially when it is low sound you may not know it is on] there is possibility of draining your battery.
I think this is not advisable. Instead you can get him a small music player for Rs.300 - Rs.400 which can play music from usb/fm and leave it with him in such situation [if he is keen on listening to music] instead if he wants only from car audio you don't have any option.
Some of interesting MP# players available online and all are below Rs.500