Yep wala ka na kasi magagawa sa kotse, isa lang talaga ang listening position niya and that's the drivers seat (unless may driver ka, dun mo I-optimize kung saan ka uupo). You only have inches to move the speakers and most of the time, no way to move the speakers.
On that note, what I do is increase the volume of the right speakers so balance ang left and right with respect to the drivers position, and medyo malakas sa likod kaysa harap. So the driver always has the sweet spot.