This is unlocked in Career mode;
You must amount €1,000,000 worth of damage before you destroy your car.
(Damage is indicated by a spanner icon, at 100% damage your car will blow up)
The best way to total 1,000,000 is by causing as much mayhem as you can (crashing into other drivers and destroying their vehicles, smashing signs,fences etc.) This is done easily by using the 'Heavy Truck' from the Road Chief class and sharply turning it sideways while pressing (handbrake) on the Autobahn so you crash sideways into oncoming traffic while the poles on the trailer unhinge and smash into cars causing extra points of damage.
If you are having trouble with this, note that you CAN repair your car as many times as you like before your damage reaches 100%.