My reverse light switch on my 2003 TD5 is faulty and the and the new reverse switch has two black wires with no positive or negative shown on it. How do I know which one is which.
No you dont as its a Normally Open (NO) switch, when it closes (ie when in reverse) it closes and forms a circuit. There likely isn't a live going through it.
It just joins the wires together and breakes them when not in reverse.