Flex manufacturing allows a company to produce several models of cars on the same assembly line and to quickly ajust to demand.

Basically, this is how it works:

They build the 300, Charger and Magnum all on the same line.

The Japanese generally start out at one end of the plant stamping out the sheetmetal.
The Big 2.5 usually stamp the sheetmetal at feeder plants and ship the metal to the plants.
The metal pieces are placed in jigs and the pieces are usually spot welded together. (glue and tig/mig welding are also used)
The spot welders are sometimes on the end of the robot arm or alternatively the robot has tooling on the end of the arm
to hold the sheetmetal pieces. The robots are programmable and so are the welding controllers.
When you place the pieces in the jig a sensor will input to the robot or PLC(Programable Logic Controller, which is a type of computer with industrial inputs and outputs) what type of part it is. The robot and weld controller will then perform the
program(often a subroutine) for that type of part. Since the robots are flexible you can do more than one model in the same stations and if you want to add a model such as what they are now doing with the Challenger you add jigs to fit the parts
and add new subroutines in the robot for these parts. The new welding robots Chrysler is using also can change the end tooling thus increasing the flexibility.

The same thing in paint. You program the robots to paint a pattern for that specific model. Sometimes bar codes are used so the equipment will read the barcode and know what model it is. RF is also becoming more common. (engine assembly lines always use RF to tell the equipment what type of engine it is)

Assembly is not as automated but you still need to put the right parts in the right vehicles. All the platforms need to fit on the carriers or you need to have different carriers for different models.