Brake lights out

Did you check/replace bulbs? check sockets for condition?check grounds?

switch in your steering wheel that's your blinkers run off of

Not yet. All brake lights are out but the running lights work. I’ve read also if the third bulb is out then all are out. Is that true and which is the third bulb? The back window brake light?

My blinkers work.

It’s just one brake light on passenger side, it works when it wants. I’ve replaced bulbs and checked wiring harness. What else should I do? It’s a 2006 Hyundai Accent.. please someone let me know, thanks.