It appears that not only does the requirement need to be a car from the Cult Classics category, it also has to be a car that is currently on sale during the #Forzathon. I first attempted this challenge using the Meyers Max barn find, the challenge did not pop. Then I attempted using the '73 BMW 2002 and made sure I counted every time I did a wrecking ball skill, again at 15 it did not pop. Both of these cars are listed as cult classics. Finally, I went to the Autoshow and purchased a car on sale, a 1960 Chevy Corvette and after performing 15 Wrecking Ball skills the challenge popped.
The wrecking ball skill comes up after performing several wreckage skills in a row, I believe it goes from Wreckage to Great Wreckage, Awesome to Ultimate and finally Wrecking Ball. The area I used to get the skill was the 'farm' of palm trees just to the west of the South Surfers Speed Trap. There are rows of these trees and just keep mowing them down till you get the wrecking ball skill and repeat.