The song is 'I've Got The World On A String' by Arlen/Koehler.
I’ve got the world on a string
I’m sitting on a rainbow
Got the string around my finger
What a world, what a life - I’m in love
I’ve got a song that I sing
I can make the rain go
Any time I move my finger
Lucky me, can’t you see - I’m in love
Life’s a wonderful thing
As long as I hold the string
I’d be a silly so-and-so
If I should ever let her go
Just a guess, but it sounds like another excellent cover by Pamela Miller, who sang the kickin version of 'I Only Have Eyes For You' in an earlier Mercedes commercial.