Well, let's do a bit more realistic than digimods one..

We start with a toyota pronard here.
http://www.nscape.org/slicky/windtut/pronard1.jpg

Make a selection of the window to be opened
http://www.nscape.org/slicky/windtut/pronard2.jpg

Layer via copy it.
http://www.nscape.org/slicky/windtut/pronard3.jpg

Select an area to be without the glass, then adjust it's contrast and brightness when it looks nice. In this case about -55 and -30
http://www.nscape.org/slicky/windtut/pronard4.jpg

Hardest part. Invert the selection ctrl+shift+i and then use polygonal lasso with alt pressed down to unselect the main area of window so there is only a bit selected. Grab the dodge tool with about 32% exposure and a large size.
http://www.nscape.org/slicky/windtut/pronard5.jpg

Dodge the whole selection so it looks ok. Then grab a smaller dodge brush with bigger exposure to make a highlight.
http://www.nscape.org/slicky/windtut/pronard6.jpg

You should end up with this.
http://www.nscape.org/slicky/windtut/pronard7.jpg

nice, pinned ;)