I know what the light checker does but not exactly how it does it, lol. If you study a wiring diagram for your model, you will see you have the more complex 7 wire light checker. The R/W wire doesn't come from the fuse box, it comes from the starter/headlight safety relay. Instead, there is a brown wire from the fuse box that supplies the power. The R/W wire supplies power as well but only until the engine starts, then the safety relay cuts it. Like I said, I don't understand how the light checker operates. The power on the R/W wire may not be powering anything in the checker, it may be just monitoring it to tell when the engine is running.