The cutaway of the door doesn't really look like it follows a rear fender ... that makes me think that if it is a rear door off a phaeton, it might be from quite a large car.
What are the two black things at the top of the door, a couple of inches in from the sides? They could be clues.
Any chance of including a pic of the inside of the door? That might help too (wood frame, steel frame, glass channel, etc)