First. There are two different types of switches, the less common is a box of many buttons.. (Matrix Switch) if you had that you would likely not be asking this question as it is well labeled (Selects ANT/Cable/DVD/SAT/DVR/AUX for each tv independent of the others)
The more comon one is a wall plate, 12 volt outlet, antenna connection, switch and light.
It is usually near both the antenna and the "Main" tv (TV in the great room) but I've seen some strange locations.
It may be in a cabinet, or cuppboard or some other hidden place.. ON one THOR model it it is hidden by a wall wart that pwoers the JACK antenna's Sur-Lock piece of junk signal strength meter. I do suggest an upgrade if you have that switch (Winegard Sensar Pro).