Only the initial position of the attachment object matters. It will grab the vertices within its volume and make them attached to one another, even if they are in separate objects. If your car is not a Lucid particle object (for example, if it is a collision object), then you might be better off by using a spline for your rope, then selecting the end vertices and applying Linked Xform to them which would be linked to your respective cars. Hope I understood the scenario correctly.
Marsel Khadiyev (Software Developer, EPHERE Inc.)