It sounds like the flow control valve (thermostat, if you will) for the washer resevoir has failed or is stuck open. You will have to remove the heater coil from the resevoir and test it. Simple to get out. Open main fill cover and at the bottom of the coil is a little locator shaft that fits in a recess in the resevoir bottle. Pop that out and remove the coil with the main cover. Make sure engine is cool and cooling system is not under any pressure. You can bypass the heating coil by connecting the inflow and outflow hoses together. Test the control vavle by running cool water thru the inflow side, (should pass freely). Boil up some water in a pan, dunk the valve in it and wait a minute or so. If it's OK, no fluid should pass thru the coil. If it does, you will need to replace the valve (thermostat) or the entire coil. Hope this helps.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.