I'm not at all familiar with the E320, but if you are having problems with both your oil pressure and quantity gauges and a random draw on the battery, I would definitely look into the condition of the lower harness. You could probably pull up a picture of both harnesses (E320 vs. E420) and compare them.

On the E420, the big wires leading to the alternator and starter are really heady duty and it's hard to imagine that they would deteriorate substantially. But the smaller wires for the oil pressure and quantity gauges as well as those going to the starter and alternator were definitely shot on mine and the car wasn't even driven in a winter climate. It is not hard to get under the car and check these wires.

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