Since the Nissan is one of the most popular cars around, I decided to use a different approach to modeling the interior wall of the car in Gmax. Instead of copying the exterior surface and flipping it, I decided to clone and flip ONLY the roof to make the ceiling. After that, I used the same technique on the rest of the wall that was use to model the surface of the car. Model it one poly at a time.
The trick worked. I managed to create the walls and put in the back seats without any complications. I even got the walls to correllate with the doors so that the mesh can be nice and smooth at any angle.
All that's left now is modeling the dashboard, front seats, and center console. Now that's where the REAL tricky part will come in. Once those parts are done, I'll get to work on the small details of the model [suspensions, mirrors,levers, rims, etc.].