1988 Chevy Silverado tail / brake lights / turn signals

Hello, your lower brake lites and turn signal, use the same bulb filament at the rear, just not at the same time. They are wire through the turn switch. Your tail-lites are on a different circuit.

The yellow wire at the left rear bulb socket is for brake or left turn. I'd have to check the actual voltage reading, at the back then at the front. At the front, the wiring harness under the dash that goes into the steering column, look for yellow wire, hopefully there is only one, that should be the same wire that goes to the rear. Check the voltage reading on that one also, with the brakes or turn activated.

I can't tell from here, what is too bright? The tail lites are wired through the headlamp switch.