Quote:
Originally Posted by FireRanger
You just need the right LED. Get a 12 VOLT LED. Connect the + side the fan's positive power wire. Connect the ground to any ground. That all you need to do.
I did this and took it a few steps further. I have three LED's:
* AC Compressor Demand
* Fan Power
* Fan Manual Disable (switched off)
This way I always know what it is doing and what it should be doing.

That, or some simple calculations will show you what resistor you need so you don't need to pay $5 for a "12 volt" LED (which is just an LED + internal resistor anyway).
LEDs light/blow because of current, not voltage. I used this same technique with my shift **** mod and it's working fine. Simple eletronics:
Voltage = Current * Resistance
Figure out what typical voltage is running for the wire you will be monitoring. Let's say it's 12 volts as it would be in this case.
Now look at the specs for the LED you bought. There should be a forward voltage drop and a current rating. The voltage drop shows you basically how many volts are "lost" to the diode in the LED (I am not an EE, maybe someone else around here can explain that one better). Just understand that your target voltage is your source voltage MINUS the voltage drop. EG:
12 volts and 2.2 volt drop = 12  2.2 = 9.8 volts
From here, look at the current rating of the LED. Should be something like 20 milliamps (ma). Now apply it to the formula:
V = IR
12  2.2 = 0.02 * R
(Note that current (I) is expressed in whole amps, hence the decimal). Now you can figure out how many ohms of resistance you need to safely drive that LED:
9.8 / 0.02 = 490 ohms
So find a 490 ohm or higher resistor and you should be set. Note that I used a 1K ohm resistor in my shift **** because I wanted to dim the LED more (it was a superbright green LED). I usually round up a bit as a safety factor (what happens when the circuit is 14V? A car power supply is NOT the most steady thing in the world)
Depends on how handson you are with electronics. May be easier for you to find and buy a 12V LED... I just mention this as an option.