Brake Light/Rear Turn Signal Issue

[dz1087]

Just found out I have an issue with my rear brake lights/turn signals. The problem is when the right turn signal is on, and the brakes are applied, the left brake light turns off. Otherwise, everything works as it should. Both brake lights come on fully when the brake is applied. When the lights are on, the rear lights come on fine. Also, when the lights are on, and the right turn signal is activated, and the brakes are applied, the left parking light coil (the dimmer of the two in the rear bulb) will stay on, but the brighter brake light coil will not illuminate.

No issues when the left turn signal is activated - everything works.


Specs:
AAW wiring harness, installed last spring.
Bed center crossmember is grounded to frame.
Frame is grounded to engine.
Engine is connected to battery negative terminal.

Thoughts? Is there an easy way to determine I need a new turn signal switch vs a ground issue?