I replaced this on my '95 E320 Wagon as I had the same problem; it's pretty straight forward and is not part of the main engine wiring harness. Tell the parts guy to look for the positive battery cable harness as this is the main large wire and how it is listed in the EPC. One end is snaked to thru firewalls to the battery. Near the oil filter on the false firewall, there is a plug connection. It ends up at the starter and one of the wires is connected to the oil level sensor on the starter side of the engine. If you can't find the part number, I'll try to find it in my receipts when I get home. I think this part cost $60 4 years ago.

__________________
as ever
Paul

1989 560SL
1987 300SDL
1987 300TD
1983 BMW 633CSi
1972 280SE 4.5