Problem is fixed I forgot that I removed the instrument cluster because my seat belt light was flashing briefly now and then. So I pulled the seat belt bulb along with its holder out of the cluster, and just left it hanging. Mistake!

As the wiring diagram shows, the power to the radio, cluster, and turn-signal stalk are related
http://www.benzworld.org/forums/attachments/r-c107-sl-slc-class/329133d1283649174-help-original-radio-wiring-81-380sl-new-bitmap-image.jpg.

So when I reinstalled the seat belt bulb and holder, the problem was solved.