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UVA rays are constantly present, no matter the season or the weather. If you think you can't get sun damage on a cloudy day, tell that to the UVA rays. They are so powerful that they also penetrate some clothing and even glass. (When was the last time you applied sunscreen before getting behind the wheel?)
Source: about.com article, UVA and UVB Rays
Does it mean When was the last time you applied sunscreen before getting into the sun?
As Lascivious Grace states, getting behind the wheel is driving a vehicle. That's because it's actually getting behind the (steering) wheel.
Note the previous sentence of your paragraph.
They are so powerful that they also penetrate some clothing and even glass.
The author wants to convey that no matter what, these rays are everywhere. If you think that you are merely driving a car and thus safe, it's not so. The rays penetrate the glass and affect your skin. The author is asking and indirectly advising that applying sun screen is better even though your plan is just to drive.
Behind the wheel is an idiom that implies driving a car.
behind the wheel (also at the wheel)
driving a vehicle
Mia was behind the wheel, and Kim was studying the map.
Source: thefreedictionary.com
Thus, in your paragraph:
the last time you applied sunscreen before getting behind the wheel
simply means the last time you applied sunscreen before you went on a drive.
"Behind the wheel" means "driving a car." The wheel in that saying refers to the steering wheel, which the drivers sits behind. The label is saying that you can still get sunburn even when inside of a car.
>before each line. You may find many more formatting possibilities by clicking on the?button at the top right of the box in which you enter your question. – StoneyB on hiatus Aug 25 '14 at 13:02