I got an idea ... Clean the side of the truck body with the Delete tool (scissors) . Then, use the Polyline tool to draw your window directly on the body . The vertices you draw will "stick" to the polygon of the body, whatever its shape is . Then, select the two polygons (you should get something like on the image), empty them and loft them together . Then, you can fill the window and use the add thickness function or the extrude function on it . This technique can be used for the windshield and the headlamps aswell, i think . I hope it helps . litst