usually and typically the flasher unit senses the current consumption of all (good) lightbulbs if there is a burned out bulb, the current is insufficient to turn on the pilot light in the speedo cluster (that is if the speedo cluster bulb itself is not burned out)
I've had a bad USB cable shutting down the iMac. When trying to restart, it would begin and then shut off again. After unplugging the USB cable everything OK.