If it's an older car the door frame will be easier to tweak but you can close it with a dollar bill stuck between the door and the jamb. With the window down, start at the farthest forward and lowest point. Work the dollar in and out as you slide it along the weatherstripping. When you find a spot where the tension releases you have found where the air is getting in. You can tweak the frame a little bit, replace the weather stripping, or try to lube the seal and make it pliable again.