Ironic reference to a cheap and/or beat-up banger of a car. Originated in response to Ronald Reagan's exaggeration of a Chicago welfare fraud case in which $8,000 was received. Reagan reported the amount as $150,000 dollars, possibly conflating it with the case of Dorothy Woods and her 38 pretended children, and said the woman owned a Cadillac.
Yep, this old banger's my Welfare Cadillac. Who knew they still had an Edsel in stock?