Why does the audio on my 1990 Accord radio only work with the headlights on?

I have a 1990 Honda Accord. The stock (let me repeat that, stock) radio worked fine, now the audio only works with the headlights on. The radio functions in every other way except for the audio. The display dims as it should when the lights are turned on. What I'm looking for is someone who has or had this exact same problem, with the exact same car, and what you did to fix it. I have checked all the fuses. Again, please contribute an answer if it meets the above requirements.

I drive with my headlights ON always. I wished all cars did that automatically as required in Canada. Too many not so bright drivers refuse to turn on their lights at night, dusk, dawn, rain, fog, dark parking locations, tunnels, rain, etc. Two weeks ago, in the dark morning, some dude was driving with his lights OFF. I had to make several attempts to gather his attention. I guess he had super vision to be able to see in the dark. The invention of Daylight Running Lights (DRL's) seems to contribute to this issue. Otherwise, what you wish to do can only be done by a trained and certified mechanic. Someone did something to make this happen. Why make it worse?

One of the power wires you used with i assume was an adapter is actually the dimmer circuit from the lights. It originally dimmed the display at night. Your now using the dimmer circuit as i'm guessing your 12v keyed line. If the memory also doesn't work, then its not the (radio side) red 12v wire, but the yellow 12v constant.
a lot of aftermarket radios don't have a dimmer function and some work differently than your stock stereo.
as long as everything is working as in fade and balance, locate and repair the power supply in the adapter. Most people who have installed a radio in a chevy truck have ran into the same problem.

I think the easiest solution is to drive with the headlights on. Most new cars do this anyways, and it does seem to prevent accidents.

I had a old Datsun pickup that had the wipers turn on everytime you hit the brakes hard. I just learned to live with it. Your problem seems less irritating than that.

It does not make sense because if the Radio/Cassette Unit is stock, the only way that I can see is somehow the illumination wire get the power feedback from the headlight switch or the wires rubs/chaffing/shorting somehow. That's the only explanation if the system is stock and nobody ever touch it. No modification, nobody ever remove the stock unit. Still, it almost impossible after looking at both stereo system and headlights wiring diagrams. See the included diagram for the stereo system:.
Edit: Read this thread and it may give you some ideas. Hard to believe after more than 20 years and nobody ever touch/ remove the Radio/Cassette unit.