The first thing you need to do is create a lip of polys for the front of the car. Do this the same way you did it for the hood.
Select the edges shown below and chamfer them by 0.1. You may model the inside of the trunk if you want but for the sake of time, we will not be doing that in this tutorial, especially since the trunk will be closed at all times.
Create the cut shown in red all the way to the back. Now select the polys the thin row of polys you just created and extrude them by 2.5.
Now chamfer the edges shown in red.
Now we want to detach the doors but there are some things we need to do first. Create the cuts shown below in the side of the car.
Now rearrange vertices to form the shape of the door. Select the red polys below and detach them, name them "Door". Hide the rest of the car, leaving just the detached door visible.
Create the cuts shown in red so you can form the shape of the window. Detach the polys that make up the window glass and name them "Window Galss" then hide them for now, you'll use them later.
With all the door polys selected, hold down shift and drag it a little distance in the x direction. Flip the normals of the cloned polys so they are facing the other direction. Now you have to create polys that join the two sides of he door so go around the whole edge of the door and window creating polys that represent the thickness of the door. At the end you should have a solid door shape. Select all the edges shown in red below all around the door and window on the outside and inside and chamfer them by a small amount (say 0.1 or 0.2). Whatever value you decide to go with, make sure you remember it because that is the same value you will use later. This setting will determine what the door seams look like. I used 0.2 but it's a matter of preference.
Ok the door is now finished for now. You will add the trim and door glass later. Now hide the door and unhide the body of the car. Select the edges that outline the door and extrude them inwards a small distance. Chamfer the resulting edges by the same amount that you chamfered the door and that will create a nice seam. Perform any vertex cleanup that's necessary. Unhide the door and it should look something like this:
Now you need to create the rear side window. Hide the door again. Create the cuts shown and then reposition vertices to form the shape of the rear side window. Detach the polys that make the inside of the window and extrude the edges inwards and chamfer like you did for the door window. Do any vertex tweaking and cleanup necessary.
At this point this is what you should have. You will notice that the two side windows do not line up perfectly but this is ok because you will fix that when you put the trim around the windows.
This is as good a time as any to do the insides of the wheel arches so you can get them out of the way. You should also go around the whole car and give the panels some thickness. Just do some extrudes inwards along with some chamfers as well as some vertex tweaking until you have something like this. You can put as much or as little detail as you want.