Where is the charge located in a polyatomic ion?
Great question! I like that you are thinking this through.
The charge isn't located anywhere in particular, which is why when we draw dot-and-cross diagrams for polyatomic ions, we draw square brackets around them, and put the charge there.
Remember charge is simply an imbalance between the number of protons in the nuclei of the atoms, and the number of electrons in their shells. Each is a positive or negative charge, distributed all around the polyatomic ion in the nuclei and in the electron shells.