Defective evaporator core requires removal of the dash and center console.
Quite a project, the dye stains on the evaporator core provide confirmation of the leakage.
Although this part often fails from age, its cheap insurance to keep the cabin air filters clean and free of dirt and water, water and debris will enter and will cause corrosion to the evaporator core. Although a V70 is pictured here, the procedure is the same on the Volvo 850 and S70 models.