Forward TV and sound system are probably plugged into the same outlet. That outlet will typically be de-energized whenever the ignition switch is on. This is a safety feature so that the driver cannot be distracted by the TV. Make sure you don't accidentally have the ignition on for some reason. Start with the circuit breakers, making sure to flip them both ways as they can be tripped and you can't tell just by looking. The breaker panel can be hidden in a variety of places. Look inside cupboards, bathroom cupboards are popular places, and even cupboards under counters.
KEn