I bought everything to make this tool at my local Radio Shack. The LED is an automotive LED with a built in resistor. I soldered extra wire and added the alligator clips. You're gonna' love this tool!
The LED is polarity specific, so you have to connect the red alligator clip to the voltage source indicated in the FAST TEST article and the black alligator clip to the control source (be it the ignition module/igniter or ECM or ground).
Do not substitute a regular 12 volt lamp (Test Light) where ever a FAST TEST article calls for this LED test tool!
Radio Shack of course no longer sells these LED lights, but Fry's Electronics does! You can buy an LED with built-in resistor on their online store. Here's the link: Linrose 10mm Jumbo Panel Mount LED - Green B4383H5.