The brake light ECU is really only waiting on the message from the brake system ECU. The brake light ECU intercepts that broadcast message, but chooses to ignore it because it has no relevance. Whenever you blow the horn, the horn ECU sends a broadcast message out on the CAN bus network to every ECU it is connected to, including the brake light ECU and the braking system ECU. Let's imagine there's one ECU that controls the brake lights, one ECU that controls the car horn, and one ECU that controls the braking system. Every ECU intercepts every broadcast, but individually decide whether or not to react to it. Essentially the CAN bus is a bunch of linked ECUs within the vehicle that communicate with each based on a broadcast. From Wikipedia, the Controller Area Network (CAN) bus is a 'vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle without a host computer.' These devices can also be referred to as electronic control units (ECUs).
