This seems to be a variation on previously reported (also via Facebook - go figure) incidents where cash or other items were placed under windshield wipers. It was alleged that the intent of this was that the driver would not notice until they were in their car and had their car started/key in the ignition. If the person were to get out of the car under those circumstances, they were vulnerable to a quick carjacking. It seems a shirt wrapped around the wiper is a bit overkill though, and likely to be noticed before the driver gets in. On the other hand, most people won't drive around with something like that (you could easily drive away with a $20 bill under your wiper), so it would likely cause a delay, making the driver an easier target to approach, perhaps for a traditional robbery?
Mike