Exactly. When you have no differential to the wires, ie: both measure 12V, the fan doesn't work. As soon as one of the wires goes to a ground potential, then you have a 12V powered motor.
Blue wire is 12V all the time, the black wire goes to ground when the temp sensor switch grounds.
Put the LED positive lead to 12V, and hook the negative lead to the wire that goes to ground when the fans kick on.
Just think backwards. Instead of giving voltage to kick on the fans, you are introducing a ground.
Blue wire is 12V all the time, the black wire goes to ground when the temp sensor switch grounds.
Put the LED positive lead to 12V, and hook the negative lead to the wire that goes to ground when the fans kick on.
Just think backwards. Instead of giving voltage to kick on the fans, you are introducing a ground.