Move the edges on the door where they need to be in relation to the actual place where the door is not connected to the body. If you have to, make a split in the area.

Then select the door panel and extract it. Then select one set of edges like for the front most part of the door and move them back just slightly to show a gap. Then on the same edges, extrude them in a tad to show "thickness". Do the same for both sides of the door (front and rear) as well as the sets of edges that you extracted away from.

Then in sub-d, select those same edges again (4 sets) and perform a couple creases on it to pop them into shape.

Car looks nice!