300SD Electrical System Dead

My 300SD won't start; no power to anything, lights, starter, dash
lights. I briefly saw the dash lights blink, then heard a relay (?)
click and then nothing. Same with the headlights. So, I pulled the
battery out and got it load tested at the local auto parts store. It
had over 14 volts DC with no load and even with a 200 amp load it still
put out 11 VDC. The parts guy noticed their battery load tester getting
warm and smelling like burning electronics, so the battery is good.
That saved me from replacing a good battery but now I'm baffled .... Are
there any common problem areas with this model (126)? Does anything
very early in the car's wiring (past the battery) typically come loose
or corrodes? I tightened the negative battery cable but that didn't
help.

Any advice appreciated,
with thanks,

Robert W. Roe
Bethlehem, Pennsylvania, USA