It should be right on the flat metal section in front of the radiator. It's usually a 3-digit code with a bunch of other numbers. And if you can't find it there, usually you can just get the right color from the paint manufacturer. I imagine there was only 1 white for that year and model.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL