1978 - I haven't found much info on this contest (see full advert)
Maybe this is the same contest? Quote:
Betty Staufenbiel ... also won a contest requiring her to describe in 25 words or less what 7Up had in common with Datsun, the car company now known as Nissan. Her winning entry was: "Datsun and 7Up, a remarkable pair. Refreshingly different with budget-wise flair. From beginning to end, each the 'UN' — 7Up the UNcola and the dependable DatsUN." She was awarded a new Datsun, which she gave to her daughter who needed a car.