What is the ruling of the Sharia regarding the sponsor withholding his worker's salary as compensation for a traffic accident caused by the worker, and what should the worker do to reclaim his right?

If you are not responsible for the accident, then you cannot be held liable. If you are a guarantor, then it is your duty to pay what is due from you, unless you are in straitened circumstances, in which case you should be granted a delay. You are permitted to retain from your salary what you spend on your family. In either case, your sponsor is not entitled to withhold your entire salary.