Why doesnt it matter where a switch is located in a series circuit?
Anyplace there is a break in a series circuit will prevent the movement of charges out from the voltage source, thru the circuit, and back to the voltage source.
Visualize a series circuit: a voltage source, 1 wire connecting the source to some components -- like lights and a switch all connected in series -- and another wire connecting the other end of the series components to the other end of the voltage source. Note: there are also short wires making the connections between each of the series components and its neighbors.
If there is a continuous path providing a way for electrons to flow out of the voltage source, into the collection of components, and finally make it back to the other end of the voltage source, there will be current.
If such a continuous path does not exist, the electrons will not, can not, flow. If you can not trace the path with your finger, current will not flow.
This question is about where the switch can be in the series of components. Perhaps you do not really understand what a switch is. This simple sketch from instructables.com should make it clear.
Turning the switch OFF would have the same affect as disconnecting the wires connecting the switch to its neighbors.
With the switch open as shown, you can not trace the path for the electrons. Turning the switch ON accomplishes moving that line that is at an angle down to make connection between the 2 small circles.
I hope this helps,