i agree with it been decided on a montly vote based on innovative mod best pic or watever has been done, like jake said the car that has moved on the most in the month is a good idea aswell. a poll is a great idea as it gives people chance to vote for what they think is the best and which car deserves 'car of the month' therefore the winner is fair as they will be the one with the most votes!
basing it on a monthly theme may not be such a good idea as some people may not be doing that certain thing with their car and may never choose to be....