Gargamel hates Trash day due to the big trucks and all the barrels that things could hide behind. He would not want to go for a walk, and I used to try and ease him into it and act like he was a baby and I was trying to coax him, it didn't help. Going outside and ignoring him when he starts to get scared and upset and just making him move foward and just WALK, it took me about 3 months of trash days, but he is now totally fine. It just takes time, and like everyone said, don't act like its a bog deal, just act like its the same thing he sees all the time and treat him the exact same. Kinda like when a kid falls and everyone asks if he is OK and then they cry cause they think they should be crying if everyone thinks he is hurt.
Could you take him to a high traffic area and hang out with the windows open in the car? Thats helped me out with my boy too. Oh, I also Went to a truck storage lot near my house and walked him through the trucks that were parked. Got him over his fear pof big trucks but now looks for cats, cause there were a billion cats under the trucks and I didn't realize til like the 4th cat, that kinda backfired, lol. He can't pass a truck or van without looking under it now, lol.