Nice 'Dub...

For reference, modern VW dimmer wires are pulse width modulated 12V, not analog. According to my yummy new oscilloscope, the frequency is 83Hz, and the positive pulse varies between 15% (min brightness) and 95% (max brightness). The frequency doesn't seem that stable though - I saw values +/- 1Hz. I guess it's not crystal generated.

On a side note, RadioShack over here now sell 5mm LEDs that match VW blue perfectly. The wavelength is 430nm. Shame they don't sell any 3mm version.