Let's make the tarp that's put over the retracted roof. This is just simple polymodeling. Start with a plane at the center of the car and build outwards as shown.
Now you have the basic shape, we need to detail it. Put a symmetry modifier on it to make the other half and then collapse the stack. The reason we do this is because we don't want it to be perfectly symmetrical as that would be unrealistic. From now on we are going to work on the tarp as a whole. Make the cuts shown in red and adjust vertices to shape the tarp a little better.
Make the green cuts and vertex adjustments as shown. Make sure you make vertices look random. Select the edges shown in red and chamfer them by about 1.5 or so.
Add a "Shell" modifier to the geometry. The picture shows the settings that worked for me. After that, collapse the stack again so that the shell modifier becomes part of the geometry. If collapsing converts the geometry to "Editable Mesh," just right-click on it and convert it to an "Editable Poly" and then you should be back on track.