Sorry for my confusion about which car. I think you're telling me that the battery has not been disconnected recently on the C280. I think you are also telling me that the LCD display on your radio was working the last time the radio was working. These displays have been known to fail.

I just turned the ignition on in a car that has the same radio. As soon as the key is turned, the red LED stops blinking (the radio is off, and the power has not been disrupted).

So yours behaves differently, even before you turn the radio on.

You also say that the lights come on when you turn them on at night. If the radio is off, the display should not be backlit when you turn the headlights on. If you are saying it is, then the radio thinks it is on. Try this again and push the power button on the radio to see if the backlighting goes off.

I'm not going to disconnect and reconnect my battery to see if the LED keeps blinking after the ignition switch is turned on, but I'm guessing it will.

I think you should turn the radio on, ignore the fact that nothing appears in the display, enter your 5 digit code and push the SC button. It probably won't work, but it'll only take 2 minutes. We used a radio for months with no display. It unlocked and played just fine.

When I got tired of the radio not being perfect, I mailed it to some people in Syracuse (United Radio?) who refurbed it and got it back to me for about $150. At the time that was cheaper than a used one on eBay, though that may no longer be the case.

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1996 C280 289K Traded
1997 E420 167K Traded
2001 S430 240K Traded
2010 E550 4matic 80K
2000 GMC Jimmy 198K Gone to Boneyard
2003 Camry LE 196K
2011 Mazda3 i Sport 31K