Have got replacement rear sliding window for our Hiace van to replace broken one but not sure how to install it!

I had two rear sliding windows broken in my 89 Hiace a few years ago, and off memory this is how I replaced them:

Fit the top edge of the glass into the top slider, then push the bottom edge of the glass over the lip of the frame and into the bottom slider.

Let me know if this works.

Otherwise, call up Smith and Smith glass and ask them - they do it all the time.

Cheers.