Radio volume automatically increases when headlight is off

I have a strange problem on my 2010 Prius and I need advice. Right now I don't even know where to start. It started with my clock spring shorted and burnt. Since then, when the headlight is off and when the radio turns on, the volume just starts to increase until it reaches max. But when I turn the head light on, the radio works fine. The volume doesn't change. I would like to trouble shoot this problem but I don't even know where to start. can you experts advice me.

This is another story. When I brought my car to the dealer, they identified the clock spring as the problem. They want $700 for the part (genuine Toyota part) and $650 for labor just to replace the burnt clock spring. I paid $400 for the diagnostic, went home and replace the clock spring myself for $50. My car has been good to me. It has almost 200,000 miles. I am at a cross road of fixing this car as I go or buy a new car. The biggest concern is the battery. It has the original battery. The problem above is the only problem that this car has. No error or any problem that I know of. Should I keep this car?

Thanks everyone.