When testing circuits, idea is non-invasive and non-destructive inspection. Have everything plugged in as normal and don't change anything (or you may introduce additional errors into system). Back-probe connectors by touching metal-terminals inside connectors to measure voltage. Some people tear everything apart and wonder why things don't test correctly.
To test fan's power-circuit, use alligator-clips to connect multimeter negative-probe to negative-terminal of battery. Then use positive-probe to measure for voltage at various points of circuit (key ON):
- battery +positive terminal
- white wire leaving starter-solenoid
- white wire entering fuse-box
- white/blue wire leaving fuse-box (after fuse#7)
- white/blue wire entering thermoswitch-connector
- (short two terminals in thermoswitch-connector, then continue measuring voltage)
- blue-white wire leaving thermoswitch-connector
- blue-white wire at fan connector
- also measure resistance between fan-connector's black/yellow wire and chassis-ground.