Got your PM.

I can second what whunter said, a bad ground or a bad key switch are both plausible. I am leaning towards bad ground myself. Weird problems like this almost always turn out to be bad grounds.

Best thing to do is download the wiring diagram from the online factory manual at startekinfo.com and start tracing.

First thing I would do is turn to the "ground distribution" charts in the wiring diagram and verify every ground point has good connections and no corrosion. Use the "component locator" to identify where the grounds are. They will all be tagged with a W, i.e. W1, W2, etc.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech