This is essentially a snippet taken from a series of articles I've recently submitted to the BOC Bulletin.
The 408, 409 and 410s (and perhaps a number of the earlier cars - but my knowledge doesn’t extend that far) - all used the same wiper motor and switch, which was also sometimes known as the 6 Watt motor. This utilises a separate field coil and a relatively simple 3 pole 3 position rotary switch to achieve the off/park, slow and fast functions through some fairly devious wiring logic.
I attach a diagram, Fig 1, which shows how the voltages are applied through this wiring to achieve the various functions.
It should be noted 1) the motor is reversed to achieve the park/off position. 2) "fast" is achieved (somewhat counterintuitively, but take my word for it) by switching a resistor into the field coil circuit. 3) the diagram shows the rotary switch as seen from the back. 4) power is taken to the motor and thence from the motor to the switch and then back again.
I trust this will help you perceive what voltages should appear at the various connectors at park, slow and fast.
That the motor gets warm and nothing happens suggests that there is an open circuit in either the armature circuits or the field circuits but not both. If the voltages at the various connectors are correct (remember to probe both sides of the connector, also double check that you're actually getting a connection to earth when one is called for) I’d suggest the most likely point of failure is the brushes.
If you have to take the motor out do take great care to label all the connections to the motor as you do so, the colour coding on the motor wiring used to match, e.g. red/green from the harness connected to red on the motor, but at the motor itself this will all long since have faded into oblivion.
(By the way the 411s used a permanent magnet motor and the wiring and switching logic is completely different, even though the same switch is used)
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