how could move an object under the mouse on a 3d plane?
the complete simple project is on 
http://www.geocities.com/cannellag/opengl.zip

the procedure for move object in main.pas is:

procedure TFormOpengl.GLSceneViewer1MouseMove(Sender: TObject;
  Shift: TShiftState; X, Y: Integer);

begin
if ssLeft in Shift then  begin
      if clicc then
   begin
        car.Position.x:=x*sin(GLCamera1.LocalMatrix[3,2]/GLCamera1.LocalMatrix[3,0])/5;
        car.Position.z:=y*cos(GLCamera1.LocalMatrix[3,2]/GLCamera1.LocalMatrix[3,0])/5;
   end
   else
      mx2:=x; my2:=y;

   end;
end;


thank