Can anyone help troubleshoot tail light problems?

I have a 98 GMC Sierra with approximately 60,000 miles. The tail lights are not working properly anymore. With the ignition in the off position and the light switch off, if I press the brake pedal all lights work including the third brake light. When I turn the light switch on, still with the ignition off and press brake only the third brake light works. If I turn the ignition on and press brake the lights work properly. With the ignition on, when I try the turn signal both lights blink at the same time and both are dim. When I try just the light switch in the on position with ignition off turn signals don't work at all. I've checked all fuses that I'm aware of (under hood and in cab), all are good. When the brake is pressed I can feel the relay clicking. Not sure if the light switch is bad, or if there's another fuse or relay that i'm missing. Any help is greatly appreciated.

Thanks