Once upon a Time...
The rear window louvers first appeared in the 1965 Lamborghini Miura. It was partly a design element (comprising the then-innovative engine, installed in the middle instead of glass), and a functional element (providing at least some rear visibility). Since then, rear louvers have gradually become a fashion statement.
The "big break" in the rear louver area came in 1969 when Ford's chief designer Larry Shinoda made the now-iconic Mustang, Boss 302 and Boss 429 a staple, as well as the Mach 1 Mustang. It was stylish but leak out less heat into the cabin as the Mustangs had a fastback roof with large horizontal rear windows letting in a lot of sunlight. Even with the air conditioner on, those huge rear windows kept the sports cars warm pretty quickly. Since many of the sports cars also had a black interior, the heat from these rear windows could dramatically increase the inside temperature.
In the 1970s and 1980s, the fashion for louvers grew. Some vehicles came with louvers as a standard feature. The DeLorean DMC-12 had standard plastic rear window louvers. The louvers added the futuristic design to the car, but since the engine was at the rear of the car, the louvers helped prevent hot air from entering the cabin.