wheelとは車のハンドル、つまりsteering wheelのことですが、behind the wheelで”ハンドルを握って”いる状態、”運転席にいる”状態を指します。
be behind the wheel
get behind the wheel
sit behind the wheel
climb behind the wheel
drunk behind the wheel
The state Board of Pardons and Paroles rates the crime of driving while a habitual violator - which can involve a fourth DUI - in its lowest severity category, less serious than credit-card fraud. Clyde Harvey Pyles of Gordon County was sentenced to six years in prison for driving drunk in December 1990 - his 16th offense at the time - but the board paroled him in June after six months. He was caught drunk behind the wheel again in Gordon County in August.
(Atlanta Journal Constitution. 1991)