Car.cs

public class Car
{
    private readonly Radio _radio;
    public event EventHandler<RadioEventArgs> RadioTogglePower;

    public Car(Radio radio)
    {
        // omitting guard clause
        _radio = radio;
        _radio.TogglePower += OnRadioTogglePower;
    }

    private void OnRadioTogglePower(object sender, RadioEventArgs e)
    {
        if (RadioTogglePower != null) RadioTogglePower(sender, e);
    }
}