The rear windshield/tailgate wiper does not work. I just had my engine wire harness replaced, but the wiper didn't work before that. How do I determine where the wiring or the wiper motor is bad?

Also, the engine's temperature gauge in my dashboard doesn't work. The mechanic swears that he wired it correctly, and claimed that a temperature sending unit in the intake manifold (which he removed to rewire the engine) is bad and he wants to charge me. I looked under the dash but couldn't see whether the wires to the instrument gauge were loose.