There are a wide variety of solenoids that are available, however, you want to make sure that the one you choose is rated for continuous duty. In Ed's picture, it appears that he is using a 2 wire solenoid. One small wire goes from the solenoid to ground, and the other wire goes to a power source to activate the solenoid. The other way to wire this would be to provide a fused hot lead to the solenoid that is either switched on with the ignition switch, or is hot at all times. Then one ground wire would go to the switch that activates the siren, and the other side of the siren switch would have a wire to the ground terminal of the solenoid. How you wire the siren depends on the type of constant duty solenoid you purchase. In the instance of wiring sirens everyone has a personal preference as to accomplish this, and there are many ways that are correct. I have wired sirens using many different methods in the past. Whenever possible, my preference is to wire the siren using the automobiles horn as my switching method. This requires more of a working knowledge of how the horn blowing circuit operates, but is the most simple way for the vehicle operator to use a siren. A single flip of a dash switch toggles between the cars horn circuit and the siren blowing circuit. Almost all automobile horn blowing circuits operate by grounding the horn blowing relay that then provides current to the horns themselves. That is where the similarity ends, since some cars, the horns are dead when the ignition is off, and others, the horns will blow at all times.