what happens is cold coolant comes from your radiator. It goes into your engine. The heat from your engine is transferred to the coolant (heat travels hot to cold). The coolant hose leaves your engine and then goes into the heater core where it would normally be used to heat up the inside of your car via a fan blowing the hot air. If the heater core is hot and somehow cold water splashes it or something it will crack or leak. Many years of expanding and contracting will cause the heater core to do this as well. Bypass the heater core if you can do without heat. Otherwise do what you have to to replace it. I am not sure how difficult it is in a 90 teg.