I just now have a very similar problem with my 92 300E I have no lights unless the switch is turned on, no brake lights, tack, gauges, rear interior light either. I am trying to follow 12 volts from the battery through the ignition switch to the fuse panel but haven't found the cause of the problem yet.

Please let me know what you find and I'll contact you if I solve it.

Thanks

Acky