Becker 1692 - Radio AM/FM/WB bands only work when key is in off position
 

My 1997 S500 still has the original Becker 1692. The CD controls, cassette deck & AM/FM/WB bands all work perfectly when the key is in the off position. However when the key is in the run position (whether the engine is running or not) only the CD controls & cassette deck work. All radio bands AM/FM/WB are nothing but static. As soon as I turn the key back to the off position and turn the radio back on everything works perfectly.

I swapped in a spare Becker 1692 and it does exactly the same thing.

This hapened suddenly. One day everything worked and the next it didn't. Can anyone point me in the direction of what's faulty?

Thanks.