Some vehicles supply the front and rear washers from one reservoir; others have a different arrangement. Do not assume a rear washer alert means there is a separate tank. Check the manual for the correct filling point and fluid.
If the level is adequate but the rear window receives little or no spray, the problem may involve more than fluid quantity. For Sioux Falls winter conditions, use a suitable cold-weather washer fluid. Describe the front and rear washer behavior when contacting 12th Street Auto Care Center.