The ground for this electrical circuit is made by the switch being secured to the metal dash board panel. There is NOT a “wire” that makes the ground for this circuit. Both the wiper motor and switch complete the electrical circuit to ground through the metal parts of the body. For this electrical circuit, the firewall and dash panel. The metal of the entire car is the route to an electrical ground. If your wiper switch is mounted through the hole in the dash for the switch and you have secured the switch with the nut and washer to the dash, the ground part of the circuit should be completed.