I installed a replacement rear-view mirror that displays the outside temperature and compass reading, and also has the homelink feature. It was sold for the Mazda 3 and/or Tribute, but seems to be identical to the one that Ford sells for the Mustang. The Mazda mirror was much cheaper than the Ford version.
The only issue is that the sensor mounts under the hood, clipped up under the plastic cover between the radiator and the grill. Since my car is black, the temp reading shoots way too high when you first start the car, and it takes a few minutes to get air flowing by it so that you get an actual ambient temp reading.