You need to use the negative force of the current speed of the vehicle, and not an absolute number. Then it won't go into the opposite direction after it has stopped.
TVec3 zvel=GetBodyVelocity(chassis).Z;
AddTireTorque(car,-zvel,0);
AddTireTorque(car,-zvel,1);
AddTireTorque(car,-zvel,2);
AddTireTorque(car,-zvel,3);