First sign, blower motor starts to make a sound not previously heard.
Diagnosis, bad bearing? Change motor when convenient.
Second sign a few days later, motor not turning. (First really hot summer day!)
Diagnosis, seized bearing? Change motor tomorrow.
Third sign, motor spins again after passenger door slammed shut. When held in hand, motor vibrates on low speed.
Diagnosis, bad solder joints on ECC control unit?
Fourth sign, when squirrel cage is positioned in a certain, repeatable position the motor wont start. If motor is turned by hand a few millimeters it starts.
Diagnosis, one bad winding in motor, causing a bad sound on lower RPM's and no start in a certain position.
Motor replaced and everything works OK again.

HTH,
mila