Possible Clues:

• They had 'horse collar' grilles
• '50s flops
• Old bombs
• Group with the 1961 hit 'Rama Lama Ding Dong,' with 'the'
• Dealership duds of 1959
• Named for a car model, group who sang the 1961 hit formed by the ends of 17-, 26-, 41- and 52-Across
• 1950s autos
• 1950s Citations, e.g.
• Bombs produced in the 1950's
• Ranger and Pacer, for two