Get into the under-hood fuse box (near the battery) and find the radiator-fan relay.
Pull it.
There should be 4 contacts, two are the for the coil (one is +12v, one is ground)
Two are for the fan (again one +12v, one ground)
Use a DMM to find out which wires are +12 and which are ground.
You can connect jumpers to the battery to find out which ones run the fan (I think it's the top pair, but memory was never my strong suit)
Oh, if your relay has a diagram on the side, you can just save yourself a lot of time and read which prong is which. Mine were diagram-less.
Slip the +12v wire for the LED under the +12v (fan) prong on the relay and plug the relay back in.
BTW, I'm using a LED that already has a resistor in-line with it, I got it at the 'shack so it's already set-up for +12v power. Don't do this to a regular LED without a resistor and expect the LED to live.