The only time a Ford Mustang is featured in the Dukes of Hazzard universe is in the 1980 movie "The Dukes of Hazzard". In the film, a 1969 Mustang Mach 1 owned by Boss Hogg's nephew, Billy Joe, is briefly shown in the movie as a rival to the General Lee.
There were some other cars featured in the TV series, but the Ford Mustang was never one of them.