First, allow me to welcome you and Duke to K9A!!! We're glad to have you join us!!!
Fear is one of the hardest things for a dog to work thru and requires the most patience on our part. Each issue needs to be addressed by it's self and it can take A LONG TIME to get past each one of them.
When I got Neka, (my German Shepherd) she was afraid of storms and fireworks too. I worked at it a bit each time a storm roled in. I would sit on the floor and play with her when i knew one was coming. Every time the thunder hit, Id make goofy/happy noises and say "Wow, that was a big one" in a lite and happy tone and then give her a treat for staying ingauged with the play session. When she had "enough" and wanted to leave I would let her go to her "safe zone". I did the same with firework. (We have a stadium about a 1/2 mile away that displays fireworks after every home game). FYI, it has taken almost three years, but she will know sleep thru an average thunderstorm. The BIG BOOMS still send her to her safe place in the basement.
With the toy cars, maybe try having one of the kids bring a toy over and let the Duke hang out with it when it's turned off. This will serve two purposes.
1. It will allow you to see what reaction (if any) Duke gives to the toy itself.
2. It will show Duke that the toy is nothing to be afraid of.
Then allow the car to sit idling 20-30 feet away ... and so on. If Duke reacts, move Duke or the car futher apart until Duke is calm.
Then do the same with the car moving. Maybe go to a playground so you can get more distance between Duke and the car.
It's really a matter of keeping the dog calm while moving closer and closer to the target. I have an article on the "Betterdog4u" site dealing with fearful dogs, you can adapt and adjust your stratagies to fit your situation, but the principles are the same.