test type depends on function
try this for size:
you will test for voltage (or lack thereof) if the function under test returns a voltage to indicate a certain state
you will test for current if the function under test returns a current value to indicate a certain state.
Normally, a motor (like the window winders, aux pump, blower motor) will show a fault if there is excessive current flow (pops the fuse) whereas a voltage test will probably not show a lot (OK, I know a short on the motor winding will show 0V across the motor... but then the wiring would have smoked long time ago ...)
Anyway, remember to disconnect the device under test (DUT !) from the circuit it functions in when resistance measurements are taken.
A rule of thumb to consider:
when there is a relay or a switch or a fuse in a circuit, there should be *no* (OK, very little) voltage drop across the switching elements. A high voltage drop will indicate a high switch resistance. This will lead to less than optimum voltage to get to the part that needs it. If you end up with a voltage drop approaching 12 or 13V you have an open circuit (i.e. you test across a popped fuse).
All motors should draw less current as per fuse rating for the circuit and the voltage drop should be close to 12V across the motor (unless you are losing voltage across a bad switch, too thin wiring or a burnt relay contact).
Relays should draw little current when energised and the resistance across the coil should be rather low (couple hunderd to 3000 ohm). As said, voltage drop across a relay's contact should be minimal.
The speed sensor on some AC compressors delivers an alternating voltage that can only be measure with a scope. Testing for current on this device will get u nowhere fast.
hope 101 made some sense. I have some test methodology on soft copy if u need more info