I drive a 2000 Toyota Corolla that I've owned for 2 years. My air vent has recently started turning off by itself for a few seconds at a time. At first, it happened once per week, but is now occurring 4-5 times per day. During these moments, the other electrical functions of my car (windows, etc.) Still work, so the problem seems isolated to the vent itself.
Shortly before this started, someone tried (and failed) to hotwire and steal the car. I thought the problem might be connected to it, but found that they hadn't cut any wires.
Symptom indicates the blower fan motor is on its way out or you have aloose connection somewhere. Give the blower motor housing some banging with your hands and if the motor works better, the carbons are running out.