I have the same type of problem, getting that conversation started is like a brick wall. sigh. I would say, maybe you could initiate the convo via email beforehand, and then follow up in the car with something like, "Do you have any questions about my email?" that way, you can either talk about it in the car, or you can talk about something else, but either way, you've mentioned it, so it isn't stuck in your head.
Let us know how it goes! Another thing might be- write out notecards of what you need to say and then maybe you could read them in the car. I'm sorry if that sounds lame, but its a technique I use all the time when I know I'm going to stress out and not say what I need to say.
_________________
Your Aspie score: 165 of 200
Your neurotypical (non-autistic) score: 48 of 200
EQ 12 SQ 70 = Extreme Systemizer