The auto dimming side mirrors recieve their signal to dim from the rear view mirror. To test you can turn your lights on, make sure the transmission is NOT in reverse, block off the light sensor from the rear of the rearview mirror, then shine a light at the light sensor embedded in the rearview mirror glass. All 3 mirrors should dim.
If not, you might have a wiring break or defective rear view mirror or defective side mirror glass... need to pull the side mirrors off and check for voltage while doing the test (not sure what voltage though).