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Good afternoon, I hope I can help you.
Faults such as this is normally caused by a bad earth lead or corroded terminal somewhere.
Start by disconnecting the rear wiper wiring plug and retesting to see if fault still happens.
If the lights still come on then you do most likely have ashort in the wiring. If they dont come on then fault is probably an earth fault at the motor.
Usually you find that short circuits have no particular cause. Your particular type is worse to find because it isn't blowing fuses.
You need to inspect the harness for areas where it may have a tight bend or has been a bit loose and allowing the wires to rub together and touch through to the copper inside.
I should also say that you should ask yourself, have I fitted new bulbs recently or made any changes that could cause problems. Sometimes fitting the wrong bulb type causes problems.