While Minnesota traffic laws do not specifically mention dirty windshields, statute 169.71 says you are prohibited from driving any motor vehicle with a windshield so discolored it obstructs vision.
The same statute says every vehicle must be equipped with a device to clean moisture, including rain or snow, from the windshield. This device must be controlled by the driver of the vehicle.
If you were injured in a crash caused by another driver, and your lawyer can establish the driver violated one or both laws, he or she may be held liable for damages.