The reservoir is under the spare tire under a plastic cover. I've not personally added any, but in reading on this site people are pretty picky about what kind of fluid to add. The recommendation is to get MB fluid.

Hope this helps!

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)