The wires that go bad on the lower harness are for the oil pressure and quantity gauges. I have never heard of the large starter wire in that harness going bad.

Have you tried hooking up a charger to the battery to supplement it during starts, just to rule out a situation where there is current but it just isn't quite enough to turn the starter?

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