Move the resulting new edge to the position shown to add more definition to the corner area of the door.
Now we have to a do some chamfering to sharpen some edges. Select the edges shown and any others you think need chamfering and chamfer by a value of about 0.2 or 0.3 depending on your preference. Make sure you clean up any unnecessary vertiecs.
Now back onto the body of the car. Now we have to add the door latch. Make the cuts shown in green then delete the red polys.
Edtrude the edges shown in green inwards and then seal off the hole by creating the polys shown in red. Now you have a nice depression in the geometry for the locking mechanism.
Now comes the laborious task of chamfering the edges to get sharper edges. Select all the edges around the door area that require chamfering and chamfer by 0.2 or 0.3 depending on what you prefer. Perform any necessary vertex cleanup.
When you have chamfered the edges, you should have some nice sharp edges as shown.