What would the role of a thermistor be in the power door lock circuit
of a car? It's in series with the door lock "motor", which might be
a solenoid or maybe a rotating motor.

It might be a positive temperature coefficient thermistor, such as a
"polyfuse". These have a very sharp PTC "knee", and can be used as
self-resetting overload switches/fuses.

At a guess, this one might be selected/timed so that it would remain
cool (closed) if the door lock is operated for a second or so, but
would heat up enough over several seconds to "open" and disconnect the
solenoid/motor thus protecting the motor from overheating or stripping
a gear if somebody holds the door-lock button down continuously.

It could also protect the power circuitry from damage in the case of a
short circuit at the motor/solenoid itself.

Is it possible that it is meant to lower or turn off power to the
mechanism if the circuit is left closed for some reason and the
solenoid would otherwise overheat?

That would be my guess.

--
Dave Platt AE6EO
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