I'm not sure that I'm understanding what you mean by the big pin and junction box.

You might want to try posting your issue over at *********.com to see if those guys have any ideas. The 400E and 500E are virtually identical cars when it comes to electrical issues like yours.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles