KEY WEST, Fla. — The all-new Cascada is the first drop top to adorn a Buick emblem since the Reatta convertible whose production ended in 1991. Sales were dismal, with fewer than 2,500 units sold, when Buick pulled the plug. Cascada, Spanish for “waterfall” and built in Poland, has been selling under the Opel, Vauxhall and Holden badges for the past two years, is beginning to arrive at U.S. dealer showrooms now.