This next step is a bit of a jump but I'm sure you will have no problems with it. Create the polys in red then extrude the edges in green to create the shape shown. Adjust vertices etc.
Now we need to add the wheel wells because they will affect the shape of the back seat area. Select the body of the car and select the edges around the wheel arch and extrude them inwards. Extrude first a small distance then extrude again all the way into the wheel well.
Now it's just a matter of closing off the wheel well. Should be easy for you at this point. Create the polys shown in red to seal off the wheel well. You can do this any way you please, it doesn't have to look like mine. Repeat the process for the front wheel arch.
Now go back to the interior and you need to sculpt it around the rear wheel arch. Delete the polys shown in red as they intersect with the wheel well.
Extrude the green edges up as shown.
Now we have to add a little more to the body to get things to fit right so select the body. Select the green polys and extrude them twice as shown by the red arrows.