If this helps:
It is my understanding that your employer must notify you that they are or will be garnishing your wages. The employer receives a CO from the probation department ordering income withholding. They must follow the order or they face possible action for non-compliance.
If you contest the action, the battle is between you and probation, not you and the employer. And if you take on this battle, good luck, because the probation department is like a robot, simply following instructions from the court. Ultimately, you will probably have to file an application with the court to contest the action.
I recently filed an application (motion) with the court and successfully had my CS reduced. In that action, my ex requested income withholding through wage garnishment but the judge never granted or denied the request. The judge made no mention of it in the decision. Regardless, one month later I had my wages garnished and they are still garnished today. I could contest the action, but the sensible thing is to let the garnishment action stand. Ultimately, they are just saving me a stamp every week. Bottom line, you gotta pay it one way or another, so you might as well do it the easiest way.