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.

what about the extremely bright tail lights?

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

sorry, hard to explain without you being able to see them, but the tail lights are extremely brighter than normal...they look like the brake lights should...it's weird, almost like whoever owned this truck before me tried to do some wiring and somehow messed up the wiring under the dash so that the brake and tail lights are reversed...i pulled the tail light off on the truck and put a meter on each wire on the harness at the rear with the headlights on, then turn signals, then brakes...same wires were energized with all three....is that how it should be?