You can change the wiring harness on the 400E in the parking lot in about an hour. It is really easy. You have to take the air filter housing off and you will see it. The hardest part is where the lever-type rectangular plug goes through the hole in the firewall towards the back passenger side of the engine compartment, but that's not even too bad.
All you need is an Allen wrench set and a screw driver. If I recall correctly, there are 8 fuel injection plugs, a plug for the mass air flow sensor, one for each cam sensor on the front of the engine, one for the coolant temperature sensor on the front intake manifold and maybe one or two others. It is really intuitive once you lay the new harness along the old one.
If you search wiring harness and 400E or E420, you should find more information if you need it. Here's my post from years ago and some of the issues I ran into.
http://www.peachparts.com/shopforum/1516932-post21.html