I live in europe, but local vw dealer can sold you the needed additive: "Satacen 25". Fluid reservoir has 4.0 liter capasity and it is located inside of spare wheel case. The container is pressurised and can be accessed via two pipes(from where the spare wheel is located). From local vw dealer it is possible to byu kit which helps to change the fluid...
But I recommend to let the change job to the local dealer. The old additive is first taken out and then reservoir is filled with new one. The additive reachts with air and makes some crystals which can cause some problems.. so its changed regular times.
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