The surface of a car's side window is always challenging to model because it requires a perfect revolve surface to ensure successful lowering. 

Inspired by Alias, this geometry node group can provide an easy approach to creating side window surfaces. It will give you a reference for shrink wrap or snapping.

Installation

1. Open the target file where you want to use this geometry node group.
2. From the menu bar, select File and then Append. This will open the file browser.
3. Locate and double-click the provided .blend file. In the "NodeTree" directory, choose "Side Glass" and click the Append button to add the node group to the current file.

You can now use this node group as a modifier.

How to use

You need exactly 5 sample points to create the surface.

3 of them are used to describe the "vertical" arc, they should be connected as a polyline. To increase the accuracy, it's recommended to align these 3 points along the same x-coordinate.

The remaining 2 points are isolated and are used to describe the lengthway arc. 

Note that the revolve axis is along the x-axis, so please make sure the car's center line is also along the x-axis.

Apply a geometry nodes modifier to the object and select the "Side Glass" node group. You can now interactively adjust the side window surface.