My '91 300SE either had working rear fog or needed only a bulb.

[edit] I might be wrong. Maybe I'm thinking of the S420.

If you replaced the stereo, you can put a switch like Jeremy's in place of the speaker fader by the console window switches. You can also get the panel the 500- and 560SEL have by the ashtray that has the pair of front seat heater switches and the highway tone horn. Use one of the switch positions for a GDO.

Sixto
87 300D