I seem to remember that he used candle wax. You melt the wax into a sheet, then place it in the freezer so that it becomes brittle. Break the sheet of wax, and presto... broken ice. Fix the vehicle and ice in a suitable clear medium.

That's exactly right.

Here is a article that whas (before the crash) on my site

Making effective Ice

You can use candle wax for small areas, but for a large patch of ice like in Mig's dio, paro-wax (the wax used to seal bottles for home made preserves) melted in an old pot and then poured in would work much better.

Candle wax is the common method. There is one example here on Armorama, I believe Envar did it. It's a small Russian tank exiting a small river. I searched but couldn't find it.

Its a good example of the method.

I need to make an addendum to my post about using candle wax. When you pour your clear medium, make a test pour first. Many clear resins etc can be quite exothermic. You wouldn't want to melt your wax...

True enough, candle wax works for small areas. For larger, more complex dioramas use parafin.