Beyond The Exterior
Cab Interior Side Wall
When it comes to creating the interior cab walls, I advocate using some pre-existing geometry. It's surprising how nicely different HyperNURBS objects end up fitting together when you generate the surfaces from common points.
In this case, we'll learn what I mean with the rear side wall, the opening just behind the door. Select the polygons between the door edge and the rear wheel well, (or far enough back so the polygons are beyond the rear cab wall we just created). No need to select the polygons of the exterior car surface that curl in overtop the cab, just the vertical wall...
Then use the 'Point Selection from Polygons' command in the Selection menu.
Copy and paste these points in the structure menu into a new polygon object (under a HyperNURBS/symmetry hierarchy of course). The Set the X value of the points such that they are all planar and at a distance that places them right underneath the edge of exterior surface's overhang. Use the bridge tool now to create the polygons of the interior cab wall surface as shown below.
Now copy and paste the points from the car door edge into the cab wall object and bridge them to the existing wall to create the face of the wall on the door edge. You'll need to have a thin row of polygons at the interior edge to keep that edge sharp. Also an extra thin row at the top of the interior cab wall ensures the wall fits tightly up against the underside of the car exterior.
(It's important to remember as you create all these polygons to go back and select all when you are done and align the normals... as you can see from my screenshot above, it clearly hasn't occurred to me yet!) ;-)
I hope that was all easy enough. The rest of the cab walls are going to follow this general technique...