There's no direct sensor for these. They are triggered through the bcm indirectly by the range selector switch, but there's a relay that controls the lamp inbetween. I'd test for good ground, then go on to test the power coming from the relay. If that's good, I'd test the continuity of the power wire and then have fun locating the break. Might just be a bad relay though. Posted the schematic on your other post if you want to check it out. Autozone has all the schematics you'll ever need.