I have done this several times, there are a couple of important things to consider. the first is to use a relatively small aperature (I normally use at least f16, sometimes f22) This will allow you to capture just about everything in focus. Next make sure to maually focus so the inside of your vehicle is in sharp focus. The third thing to consider is placement of yourself and your mirrors. I frame it so I am not in the picture at all and I have adjusted my mirrors so neither the camera nor myself are visible in the end result. Also I do not use flash at all for a picture like this, the long shutter speed should allow plenty of ambient light to expose the inside of the vehicle.