brake light and

At the lower rear, stop and turn use the same bulb filament, just not at the same time. The blk/yel wire at rear connector is voltage for tail lamps. The solid black wire is ground, the other color is turn and stop. so if tail lamps work the ground should be ok? Your lower brake lites are wired through multifunction switch. The hi mount stop lamp isn't wired that way. The brake fuse is hot all the time. The turn fuse usually goes hot with the key on. You can use test light to check for voltage. I'd have to circuits to and from multifunction switch. I don't know if replacing multifunction switch will fix. The wiring circuit, somewhere could have a problem. Hope the diagram is correct. good luck