I was driving in the Dallas area at highway speed (approx 70mph) during rush-hour traffic when a crackling sound presented in the dashboard area from what appeared to be the sound system (Harmon Kardon surround system). This was immediately followed by the unmistakable smell of burning plastic or something similar. Smoke coming from the center speaker in the dashboard followed subjecting car occupants to potentially hazardous smoke and chemicals resulting from the fire.All signs suggested an electrical fire which was alarming considering this occurred at highway speed. Turning off the sound system did not stop the issue. My safety and that of others on the highway were put at risk because a fire of unknown origin occurred while driving at highway speeds. No accident occurred. Further exploration of the issue from a safe area off of the highway confirmed the following: 1) possible electrical fire emanating from the center speaker of the dashboard 2) center speaker was damaged and appeared melted 3) the only way to stop the fire was to remove the fuse powering the sound amplifier which allowed the car to be driven to the local Mercedes Benz dealer/repair center The car was inspected by the local Mercedes Benz dealer who confirmed: 1) the center speaker was damaged and not repairable 2) the cause of the problem was a "shorted" and otherwise faulty amplifier. 3) the cost to replace the speaker estimated over $400 4) the cost to replace the amplifier estimated over $2700 There