Measure a real seat in inches or centimeters, convert those measurements to BVE standard meters,
and use the measurements in a B3D or CSV file to build an scale object of an individual seat. Then write an assembly .Animated file to combine\duplicate\position the B3D\CSV seat objects within the car exterior object, such as
[Object]
States=CarExterior.CSV
[Object]
States=CarSeat.CSV
Position=X0,Y0,Z0
[Object]
States=CarSeat.CSV
Position=X1,Y1,Z1
[Object]
States=CarSeat.CSV
Position=X2,Y2,Z2
etc, etc...