Hit by car effect

Hey, im trying to do that AE classic hit by car effect, where you film the victim getting hit and then the car driving by. My mistake was that in between the time i filmed the vitim and the time i filmed the car the camera moved a fraction of an inch. What i have been trying to do is to draw a mask around the car and then move the mask with the car (because although the shots are positioned slightly differently the car still stays on the road but everything around the car and me looks really blurry cuz there are two of em). My problem is that when i try to move the mask i cant. How do i move it or is there a better way of doing this. I can reshoot the shots if i need to

thanks