Only read this if you have finished the whole episode!
NOTE: I have changed part of this answer to reflect some input from other alert members from this forum. This improved answer should be more correct.
here is your answer: When chatting with Doc at the jail one of the things Marty asks Doc about is the very existence of the car.
Doc's (paraphase) answer is... When the time machine was struck by lightning in 1955, the direct overload in the flux capacitor created TWO cars. One landed in the 1800's with Doc. The other was flung 70 years into the future.
Eventually Doc stumbled upon this scientific duplicate and began using it for personal/ emotional reasons.
Last edited by Gumbyfan; 12/23/2010 at 12:55 pm.
Reason: Fixed some misinformation.