Let's examine the possible scenarios:
Can you be charged with a crime of grand auto theft if someone hands you the keys to the stolen car and you drive it and stopped? YES. However, one of the elements of that crime, the state (either the State Attorney in my state or District Attorney in Arizona) must prove beyond the reasonable doubt that you had the mens rea. Mens rea is Latin for "guilty mind". In criminal law, it is viewed as one of the necessary elements of a crime. The standard common law test of criminal liability is usually expressed in the Latin phrase, actus non facit reum nisi mens sit rea, which means "the act does not make a person guilty unless the mind is also guilty". Thus, in jurisdictions with due process, there must be an actus reus accompanied by some level of mens rea to constitute the crime with which the defendant is charged (see the technical requirement of concurrence). As a general rule, criminal liability does not attach to a person who acted with the absence of mental fault. The exception is strict liability crimes.
Thus, even if you are arrested and charged with a crime, the element of knowledge must be proven.
For further assistance contact a local professional attorney in Arizona who handles criminal defense cases.

Can you be charged? Yes. Unfortunately anyone can be charged with anything. However, will the government succeed in obtaining a conviction? That's an entirely different story. Based on your comments there likely are viable defenses. You need to consult a local, qualified defense attorney to review your entire case and circumstances, and who can then advise and aggressively defend you.

When the police bring charges they are not required to decide who is telling the truth and who is lying - that is what trial is for. To charge you they need a complaint and some evidence to back it up. I assume from your post that you had someone else's car. That person says that you stole it. That is enough to charge you.

You may have defenses that can win at trial.

You need a lawyer to assert those defenses. Do not talk to anyone about this case except your lawyer. Not your girlfriend, not you priest, not you mother or friend. Not anyone. Stop posting facts about the case here.

Get a lawyer right away.

Wayne R. Foote, Esq.
Board Certified OUI Defense Law Specialist
by the National College for DUI Defense, Inc.
Law Offices of Wayne R. Foote, PA
344 Mt. Hope Ave
Bangor, ME 04401
(207) 990-5855
(207) 990-5858 (fax)
www.lawyersmaine.com
"Injustice anywhere is a threat to justice everywhere." M. L. King, Jr.

Your question leaves so much unexplained that it is impossible to answer, but that is just as well. The important thing is that if you have been charged with theft, or if you think that you might be, you need an attorney to handle the matter for you and you need one now before the situation goes any further than it has already. The earlier your attorney gets involved in the case the more can be achieved. The other important thing is that you must not discuss this incident with the police or with anybody else, including friends, relatives, and cellmates. Do not post about it again. Avvo is not a confidential forum. Good luck.