If you are testing a 94 124 body with a 119 engine than the blue 2 pole switch at the front of the engine when disconnected (open) high fan speed should engage. This signal is sent to the A/C pushbuttons, if memeory serves me correctly, and in turn controls the high speed aux fan relay.

Before going too deep, check the voltage at the fan resistor. You should have twelve volts on the one side when the blue switch is disconnected.