I just run distilled water in my washer tank. It's what the car washes spray on "spot-free" setting and works great for me. Here in Dallas it doesn't freeze very often, though; if you live up North this won't work.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280