1985 Chevy S-10 Break lights

welcome to the forum. Your turn signal and brake use the same bulb filament at the rear, just not at the same time. The problem is usually in the steering column, either the wiring circuit or turn switch. There may be a multiwire connector, under your dash, close to steering column, for circuits going to and from the column. The voltage wire from brake switch to columnv is white. The yellow wire is for left turn or brake at the rear. The dark green wire is voltage for right turn or brake at the rear. Those wiring colors should be at multiwire connector under the dash. All repair info, says to disarm airbag, where applicable, when working under the dash.