From INSIDE THE DREAM:THE PERSONAL STORY OF WALT DISNEY by
Katherine and Richard Green:
'Walt had an offer from the DeSoto car company. Recalled Ruth(Walt's sister) " They said if he'd pose in front of a DeSoto, they would give him a car. Walt said he didn't drive a DeSoto and he wasn't going to claim he did. But Mother asked, ' why not? You can give it to us!' So Walt did it and they loved that little car.'
This is one of the variations of the ad that were published, another can be found on Bob Cowan's blog:
It's interesting to note that DeSoto were the sponsors of Groucho Marx's YOU BET YOUR LIFE series.