How very natsukashii! I remember this commercial for the Nissan March back when I was living in Japan and hearing those melodic whistles.
The oh-so-breezy song, "Mike Alway's Diary" belongs to Kahimi Karie（カヒミ・カリィ）and the Cruel Grand Orchestra as Karie's debut single from December 1992. Although the spelling of always irks me a tad, one of the most famous figures of Shibuya-kei was indeed quintessentially Karie right from the start with that kittenish and breathy delivery of hers along with the happy-go-lucky rhythms and lyrics provided by Keigo Oyamada and Ken Takimi.
Within that single, there is the English/French version and right here, the Japanese version. In either case, it makes for a perfect accompaniment to a bicycle ride in the park on a sunny Sunday morning or afternoon. Plus I enjoy that guitar solo near the end to add some jazziness to the proceedings.