1985 190E full electrical system failure

[W126Ken_W.Pa]

I have a 1985 190E, and while I was at a stop light, I switched on the rear defroster, and I lost power to everything. I mean everything. No hazard lights, running lights, headlights,dash and interior lights, window motors, no power to starter, basically, everything is dead. What could be the cause and how do I diagnose it?