2003 Chevy S-10 brake lights

Yes, the brake lite switch supplies voltage to both the high and low brake lites. The lower brake lites are wired through the multifunction switch. The high-mount brake lite isn't wired through the multifunction switch. There are left and right turn fuses, did you use a voltage tester on the fuse circuit with brake switch activated? Even if the fuse is good, if no voltage on the circuit, the circuit is dead. Your lower brake lites and turn signal use the same bulb filament, just not at the same time. The problem is usually the multifunction switch or wiring in the steering column. All repair info says to disarm air-bag, where applicable, when working around the steering column.