Can I just say that your first step should be to get your car inspected? You have to start small. I go to my local shops often and ask for a drink or a chips packet. Unlike supermarkets, here you have to tell them what you want and they'll give it to you. I go there every single time whether I stutter or not and I ask for what I need. At one point, we all get comfortable with it. Personally, my stutter significantly reduced but even if it doesn't, you wouldn't mind stuttering to that shopkeeper. Then you have to expand your horizons and get comfortable going to other places and practising. Atleast when you get your basics right, you won't be stuttering 90% of the time. It will only be the 10% that you can plan for. Have an attitude where the car mechanic is just a stranger. It doesn't matter if you stutter to him.