I would start by doing a long walk to burn off the nervous energy. I would then slowly re-introduce the dog to being around parked cars. Make sure you are relaxed (instead of anticipating the reaction) and offer kibble/treats for being "good."
Once that comfort is back, do short rides, to pleasurable places (i.e. dog parks, Pet store, etc). Gradually increase the duration of the rides after that.
I agree with the advice above. I would also not make a big deal when in the car, don't be like "ohh honey shh, it's alright don't be afraid, were only going to the dog park". That does absolutely nothing except reinforce the behavior and even make your dog more anxious or afraid from your energy. This is an obsessive behavior, so I would definetly un-train this before it escalates.