BLUF: The timer only works when the dimmer is turned off.
Situation: I have a double-gang box in my bathroom that contains the dimmer (bathroom lights) and timer (bathroom fan) mentioned in the subject line. Lights and fan are controlled independently. These devices are at the end of a circuit--only one 3-wire cable (black/hot, white, red, ground) enters the box.
The devices are currently wired as follows (device to cable):
Dimmer: ground to box, black to black, white to white, red not connected
Timer: ground to box, black to black, white to white, red to red, yellow not connected.
Black and white are common to both devices via pigtails.
What have I got wrong in this scenario? The timer works perfectly as long as the dimmer remains switched off, but as soon as I turn the dimmer on, the timer buttons become completely unresponsive.