1985 Buick Skylark hello

Welcome, my wiring info on your model year is limited. Is that an aftermarket radio, any rewiring been done?

Your tail lamps at the rear, that is a dual filament bulb. One filament is for tail lamps-- brown wire. Black wire is ground, the other color wire is stop and turn. Stop and turn at the rear, use the same bulb filament, just not at the same time. I'd have to check bulbs-- bulb socket and wiring circuits.