2000 W900 Lighting question

Customer has no clearance lights on top of his cab. I checked the bulb connector up top and have a ground but no positive. He said it went out when he connected to a trailer. All the fuses I saw in the fuse box by clutch pedal checked out ok. I was looking at the switches in his switch panel and the first one has three wires. The middle wire is hot. Does anyone know what the other two wires are supposed to be? Or do these even have anything to do with the top lights? If so, I figure one is power in and one is out. What is the third? And which is which?