Police have not provided more information about the car, but after reviewing all the information, I think it is likely that the hypothesis I expressed in the forum is wrong. I think it's likely that some camera recorded a vehicle in and out of the driveway of the house (it was hypothesized by Karenky), but that the recording is of very poor quality and impossible to identify the vehicle. It might just be a blur.
Apparently, the three cars initially wanted appear in the known tape in a lapse of about 15 seconds. That could mean that the other two cars were in front and behind the Impala. One would have passed in front of the house when the Impala was ready to leave, and the other would be just behind, once the Impala incorporated to the road. For these two vehicles the Impala was identified in the recording of the gas station.
If this hypothesis is correct, if the murderers entered the car in the driveway of the house, and if the crime was premeditated, then we have very bad news. If they had parked the car away from the house, it means that they were afraid that if the car was seen by a witness, could lead to them. If they parked him close, it would mean that they were not afraid if someone see the car.
If the car was taken to the door of the house, that means, in my opinion, they had no fear that if someone saw the car, could incriminate them. This is not definitive, prisons are full of people who thought that the police would not find tracks. But I think it means that there is no direct relationship between the car and the murderers.
I think that the constant declarations of the detectives that if they found the car they resolve the case were directed more to the murderers than to the citizens. Although it might seem strange, it is likely that the desired effect was that criminals were afraid and rid of the car.
Detectives say they have investigated thousands of Impalas and have not found the one they seek. In fact, they don´t know if they have found it or not. First, they have searched between victims relatives and friends the ones with access to a Impala. Then, allegations of theft in nearby dates, before and after. Sales, fires or suspicious accidents. Color changes. Rentals.
But if relationship between these cars and the victims is not found, how to check if they are on the right track? It is a very, very difficult job. Search without knowing what you are looking, hoping to find the key sometime. Perhaps the Impala they seek, has already been investigated.