A car starter relay switch is usually located near the battery. To find it, follow the supply wire. A car starter relay switch is a small metal or plastic box that typically has four spade terminals.
If experiencing issues associated with the relay switch, check it to see if any wires are detached from a terminal. Check the terminals for corrosion, and take a look at the thin wire that goes from one terminal to earth in the car body. If this looks dirty, remove the screw or bolt holding it in place, and clean the wire and the bolt.