If you want to save some time on the wiring harness, consider threading the individual wires through the hole from the back of the firewall, rather than trying to fit the lever-type connector through the firewall hole. When I did mine, there was a brittle rubber elbow on the back of the valve cover (right side) that got in the way of fitting the connector through the firewall hole from the engine compartment. If I had taken that hose off, it would have cracked for sure. It took me about 45-60 minutes. All that was required was an Allen wrench and a small screwdriver to pry off the old connectors. If you want to be safe, take a picture of how the old wiring harness is routed before you take it off. Good luck.

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