Hittestobject on an unknown object

hi,
how can I have the name of the target object, even unknown with the hitTestObect? the function is working when i am giving the object name as parameter to the function, but i cant figure out how to find the name of the hited object…

something like this:
if(car.hitTestObject(unknown))
{
unknown.alpha = 0.5;
}
thanks