Can you add 90% rubbing alcohol to windshield washer fluids to keep it from freezing?

Yes, you can. In fact, it is the preferred method. How much to add depends on your climate. The more isopropanol (rubbing alcohol) you use, the lower the freeze point. Normally this type of alcohol is readily available in the pharmacy in 70% and 91% solutions. The cost per ounce of pure alcohol is almost exactly the same, so use the 91% for easier mixing of ratios. It's not possible to suggest a ratio of alcohol to water, since that depends on your climate. The last thing you want is for the fluid to freeze and burst the fluid reservoir, so more is better––up to 50% in extreme situations. Stay away from vinegar and ammonia because this fluid touches the paint and can cause problems.