Madkins007 says:

Most power wheelchairs work on 2 12vdc deep-cycle batteries, and often have a ‘tap’ so you can hook up accessories.

The thing to remember is that cars recharge the batteries in use, You will be draining the batteries when you listen- so you will be sacrificing range of mobility for music when you could do the music part with a few rechargeable D’s or something.

If you have a headrest, you can rig a pair of small powered speakers (like computer speakers) on the headrest to help create a ‘bubble’ of sound for your pleasure, all powered off a few cheap batteries. A rig like this would cost a lot less as well.