I'm building a Marder II D. I have a base coat of Panzer Grey and want to do a winter white wash over it. What would be the best way to do this?

Paint it white.

What effect do you want, fresh or the popular heavily worn look?

Paint your vehicle in its normal base coat, then depending on how worn you want your white you can make a thin mixture of white and make multiple thin coats in a random, cloud pattern, letting it build up in some places more than others. You can then go back and dry brush with the base coat to show where it's coming through on raised areas and edges.

Alternately, on a Panther I did I base coated an camo'ed it, gave it a coat of Future gloss coat, then mixed up some Windsor and Newton white artits oil paint thinned with Mona Lisa odorless thinner, brushed that on the model rather thick and gave it a day or two to set up. Then I went back with paint thinner and started scrubbing it off. If it isn't cured enough it'll just smear off, but if you give it enough time it'll come off in an flake pattern.

Russell,

There are lots of different methods and most WWII modellers have their personal favorite. I have used hairspray (my preferred method), warm water and a brush, as well as purpose made materials from AK and MiG. They all work well to provide any kind of winter white wash you are looking to replicate. I would do a quick perusal of Youtube for several very informative (and quick) demonstrations. I'm guessing you will find what is right for you.
Cheers,
Rick