It would seem it's either the relay or the wiring/socket where the
relay plugs in. What do the contacts look like? Any corrosion,
loose, etc? You could also identify the two terminals that go to the
fan and try putting a piece of heavy wire across them instead of the
relay and see what happens. If it the fan runs reliably that way,
then good chance it's the relay.
Also don't know the conditions that the fan is supposed to run under
on your car. But on my 300SD it only comes on when either the
coolant temp goes beyond a certain point too high or the refrigerant
temp at the receiver/dryer gets over a certain point. Under normal
conditions, neither happens.
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