Removal of early-reflection clutter is the most difficult part. A crude but effective method is to use miniature box satellite speakers on top of the instrument panel. Another method is the "jam it in the junction" mounting. Small drivers are mounted face up in the instrument panel as close as possible to the windshield and side surfaces. This allows these surfaces to guide radiation rather than reflect it.
The living room's reflected and delayed side arrivals can be simulated in a car by delaying the signals to the side speakers. Pure left and right signals are fed to door-mounted speakers with a 10-millisecond delay to simulate the IDG. This crude approximation of room reflections works quite well, but a further refinement is to add rear speakers delayed an additional 10 milliseconds. This setup simulates the beginning and end of the living room's most effective ambience-recovery reflections.
To me, the living room simulation gives the most effective ambience reproduction because it is the most natural and least like the sound of a car. However, it can be expensive and difficult to install. The add-on reverberation synthesizers and ambience extractors can give more spectacular effects. For a limited budget, a conventional four-speaker system can be optimized for both a natural soundstage and 360-degree ambience.
This classic article from the early days of the magazine may be outdated in parts but still covers the fundamentals of car audio