1989 420 SEL radio remains on and drains battery
 

The original radio still works, but sometimes it does not shut off when turning it off or even when ignition switch is off. Instead it stays on and plays and nothing can be done to turn it off.
An MB mechanic managed to make it turn off once by removing the main fuse under the hood and then quickly putting it back in.
This worked for a few days, then the same problem appeared.
I had to disconnect the radio again, and there is no code to reset this type of radio, so I would vave to go back to the MB dealer to get them to reset it again.
Is there a relay perhaps that controls the power to the radio and if so, where is it located?
THKS!