Why not just painting black or gray or combination acrylic paint (the kind sold for painting pictures, the kind that is the alt to oil paints...) over the windows... this stuff should peel right off when you want to let the winter sun in..., at least thats my understanding per what a rep once said 

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From: James Benthall <Taobythesea@AOL.COM>
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Window Tint Recommendations

> When I got my windows tinted in another vehicle a few years back, the best
> tint film
> at the time was called Luminar(?).  It was perfect (no bubbles or  such) for
> nearly five
> years until I sold the car.  If you could get the rear window tinted  without
> them screwing
> up the rear defrost lines I would do it, since that is a large area to
> expose to solar
> radiation. (Unless your rear defrost work;-)
> Mirror tint is illegal here in Texas, and I don't like the look  anyway.
> Whatever you do,
> take it to a professional and spend a few extra bucks to do it right.
> Nothing worse than
> a window with that bubbled up crap on it! (And I hear that is from using
> cheap film).
> james in houston
> 87 westy (the phoenix)
> ps
> if i wasn't currently unemployed, I would LOVE to get it done here in  Tejas!
>  all that glass
> lets a lot of heat in ;-(
> 

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