Oh, if your pc is in the trunk then I bet inside the rear bumber is a great place for it. I don't think it would be too difficult to find a way to get the wires into the bumper, if anything drill a hole in the bottom of the trunk and run the wires underneath the car.
The only reason I was thinking in the side mirror is because it is normally on the door and there are cable tubes already heading in and out of the door, and the mirror housing is plastic/ semi waterproof and convenient. Plus if you have the electronic mirror adjustmors, there will be wires/holes already made/sealed going to the inside of the mirror. It would just be a matter of finding room for the device.