I used an LED from radio shack, used some resistors because I couldn't find a 12volt one, then wired it key on 12 volt to the resistors, and the check engine wire from the ecu as a ground, then used Neptune to set a shift light using the check engine light, so my shift light comes on whenever the check engine light comes on. I can try to make a diagram to show it but it really depends on what voltage the LED you choose operates on. If you just find a 12volt you would not need any resistors. I stuck all this into the housing of a broken xs knock light, it looks good, I think that I will be making a new one soon that will be just an led no casing so I can mount it where i can see it but it will be more discreet.