Re-attaching the back wheel arch
In order to mirror the back wheel arch, it had to be detached from the original mesh. The next task is therefore to re-attach it to the original mesh so that you can continue building the shell of the car. Before you can do this, you have to remove the "Meshsmooth" modifier from the top of the stack of the original mesh. Select the first mesh and right-click the "Meshsmooth" on top of the stack. Select "Delete" from the resulting list. The mesh is no longer smoothed. Now, to attach the back wheel arch, look further down the panel under the stack of the original mesh and click the button labeled "Attach" and then click on the back wheel arch mesh. The two meshes are now one again. Deactivate the button by clicking it again.
There is a thin line of polys that runs at the top of the mesh that we need to continue. Select the edge shown (1) and shift+drag it to the first position at the beginning of the door (2), weld the vertices that overlap and shift+drag again to the center of the door in line with the edge you cut (3). Weld the overlapping vertices here too and then shift+drag again to the end of the door and once again weld the overlapping vertices. The picture below shows what I mean.
Select the three edges at the end of the door and shift+drag them to the position shown in the picture. Weld the vertices in the green circles.
Shift+drag the edges shown in red to the position shown and then move the vertex in the green circle to the other vertex and weld them.
Repeat the previous step twice until you have something like the picture below. Weld the vertices in the green circles.
Once again, repeat the previous steps until you have this:
Now just do some vertex tweaking until you have this. Make sure you also use the top viewport to line up the vertices with the blueprints.
Onto the rear end
The two edges shown in yellow used to be one single edge but it got divided into two when you made some cuts in the front wheel arch. As it turns out, this works out great here too. Select the two yellow edges and shift+drag them as shown and position the vertices as shown.
Shift+drag the indicated edge and position the vertices as shown.
The back area needs to be rounded more if you look at the top viewport so make the cuts shown below and move vertices around until the shape has more curvature.
Here are the before and after pictures from the tope viewport. Some more tweaking might be necessary later but for now this is good enough.
Select all the edges shown and shift+drag them downwards. Weld the two vertices in green circles.
Make the cut shown in red and tweak the vertex placement for a rounder shape.
Select all the edges shown and shift+drag them. Now just do some vertex tweaking until you have this. Make sure you also use the top viewport to line up the vertices with the blueprints.
Next Page: More of the rear end.