High is run through a relay to provide the full 12 volts. It is not a part of that resistor. Lower settings run through that series of resistors to change the voltage and as a result the blower speed.
Think of that little block as a series of pipes A big one that flows quite a bit, but not full, down to a tiny pipe that flows just a little. Measuring resistance , not voltage through the blower motor resistor is the way to check it.
I would suggest pulling it out and inspecting it. You may be able to tell how it works and connect an ohm meter to it for testing.
If you are having trouble with lower speeds...that is the item that gives you those speeds. The switch only sends 12 volts to this after selecting the speed. This resistor sends the power to the blower at that selected speed.
There are only three components in this puzzle. The switch, resistor and blower motor. You already said that it blows on high, so the high fan relay is working and out of the running. The blower is blowing, so the motor works. That eliminates it. The switch does work on , I assume, 0 , "3 and 4", so it is at least functioning 60%. That leaves the resistor suspect. Thus far you can not say it works in more than one resistance (3). That puts it in the lead at only 30% functional. I suggest it is the place I would focus my attention at this point.
"Speed doesn't kill, suddenly becoming stationary does." - Richard Hammond
"Speed is just a matter of Money - How fast do YOU want to go?"-Mechanic from Mad Max-
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