The problem with questions like this is that there is often several different answers. Not all are correct, and sometimes more than one conflcting ones are correct, depending on which source you hear the answer from.
The general understanding is that those with Residente Permanente visas cannot keep a foreign plated car in Mexico, as in the old days. That is not to say that you can't get a
6-month permit though. Maybe. Can will you risk driving to the border to find out? The point is, just because someone did doesn't mean you will. So before you try it, make sure your source of information is very good.