My last test I just put it on the dashboard in the sun on the way to the test. I pulled over about a mile away and attached the heat pad.
If you passed that test, you were extremely lucky. More than likely, an integrity check wasnt performed on the sample. Any exposure to direct sunlight will cause the constituents (creatinine, pH, & specific gravity) to break completely down, which would result in a "non-human" ruling.

So unless it is known for a fact that the donors sample will not undergo an integrity check, using direct sunlight to warm up the QF is not recommended.

Instead, use the car heater.