400E "Made In Mexico"
The main reason Mercedes-Benz assembled the 400E in Mexico was that this model was created almost exclusively for the United States market. The plants in Germany were in full capacity and the retooling required to build an additional spin-off the W124 series with such a small output was out of the question.
Hence, as I understand (sourced from readings in some magazine articles, chats with M.B. mechanics and salespersons), complete unassembled car kits (body parts, engine, transmission, etc) were shipped to Mexico were they were put together and then shipped to the U.S. Local parts use was kept to a minimun.
Actually, I don't know if it would help you to do some research with the VIN number and MBUSA about this issue, since as stated before, MBUSA is not to keen to talk about it. The reason? Again, anyone's guess.
Local techs told me that they have found out the 400E tend to have more minor electrical gremlins that are usually present in the W124s. Also, some problems about door aligment, and general problems about interior fit and finish are usual complaints.
The 400E fullfils a very peculiar niche created by market needs (for example: the four-cylinder, 2.0 liter, 4-speed manual transmission, cloth seats, 2-door, C124 coupe that was made exclusively for the Italian market in the early years of the W124 line-up). A 400E kept the "V-8 appeal" for the american market, without the astronomical price of the Porsche-sourced 500E. Also, you must take into account that assembling the car in Mexico also helped to LOWER the cost of production.
The W124 sedan production was kept alive in India for almost four years after production ended in Germany. At the time, a brand new Indian-made W124 would cost almost half of what the same car would have costed in Germany with the same engine and options. Weird isn't it?