This year and model has wiring harness issues.  Make sure the harness has
been replaced  It was a factory recall item.  W140s also have known problems
with the AC evaporator unit.  These have a high rate of failure.  It is a 23
hour job to remove and replace the dash the get to the evaporator....plus
the cost of the evaporator unit.  Somewhere in the $2500 to $3000 range for
the fix.  The drive trains on these cars are solid....it is all the
electronics and extras that cause the problems.  Always buy a warranty if
one is available....you will need it.
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