Following advice posted around internet:

1. Checked relevant fuses - all appear intact
2. Removed blower motor transistor - checked with volt meter and confirmed between 1400-1500 ohms (1490) resistance across leads three and four.
3. Confirmed getting ~ 10.5 volts (is this low? Shouldn't it be getting closer to 12 V) from wires leading into transistor.
4. Removed "non-functioning" blower motor. Wired directly to external battery jumper pack- motor works.
5. does not work when connected to car's electrical system.

Is it transistor, blower motor, or something else?

Voltage from the relay to the transistor should be battery voltage so that is a bit low.
Check your ground circuit. If that isn't clean and tight, the rest of the circuit won't work.

44378

. Check all under dash and under hood fuses with a test light or multimeter. Be very thorough. Visual inspection is often not good enough.

. Check the blower motor relay--sometimes, you can swap one relay for another in the under hood fuse/relay box (check for identical part no.).

. Check whether the blower motor will work on High. If it does, the problem is usually with the resistor.