The large wire with the 10mm nut should have power at all times, even with ignition key off. Let me look at a wire diagram and see if i can find out where that wire is wired into, prolly the starter or alternator harness, let me take a look and see and i'll let you know what i find.

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg