I'm completely unsure what the world of work looks like outside the US, so I'll approach this from a human aspect:
It's the "A-word" people are afraid of. As soon as you mention "autism", you're immediately on the backfoot in any negotiation because at that point you have to try to scramble to explain what autism has to do with it (why you brought it up) and in any case it just looks like you're using "the A-word" as some sort of cheap leverage to try to get your way. You're also at the mercy of people's prior prejudices about what "autism" is to them, exactly.
You can get your way without resorting to dropping the "A-word"; in fact, you're better off in negotiations without it.
In a nutshell, you can explain all your issues that you're having with them changing your schedule at the last minute, but instead of explaining that it's because you're autistic, assign the significance of your issues to something much more people-friendly more identifiable, like a "personality quirk".
Make sure your boss (or whoever is responsible for the assignment of the routes) knows you have a problem with the routes being changed at the last minute. If indeed nobody is aware that you (quite understandably) take issue with being re-assigned at the last minute, they might assume that you're okay with it. Worst case scenario in that regard is that if you don't complain about it, you might be the go-to guy when routes need changing up because they assume your lack of complaining means you're fine with it.
However, you should never complain without offering 1) A reason, and 2) A solution that makes what you want look good. I'm guessing you don't have terribly much leverage in this negotiation, so you'll want to create some by making what you want (a consistent schedule) look appealing to your boss.
Doing so might look something like this: "hey boss, do you have a minute to talk? Look, I have to level with you, my route's been changed up at the last minute every day for the past three weeks, and it's really throwing me off. I didn't say anything at first because I thought I'd be fine with it, but I've tried it out and it's just not working for me. It's just something about the way I am, I go home the night prior thinking I know what my route will be in the morning and I run it through my head over and over (lie if that's not true), and then when I find it's been changed in the morning I'm left playing a game of mental catch-up. It's just not my process for things to be changed at the last minute, is there any chance I could get my schedule more consistent so I can be a more effective driver? That would be great and I'd consider it a huge favor."
See, at this point, you've done what I like to call "playing with a stacked deck" (I'm sure there's a less flattering name for it). What you've done is made your boss aware of a problem (your scheduling issues) then offered the solution to the problem you just made your boss aware of. Management is a whole lot of stamping out fires, and you're likely to get what you want if what you want just so happens to be the exact same thing as the solution you just gave your boss to the problem you just created for them. See what I'm getting at?
When you create a problem (i.e. make your boss aware of a problem) you can easily maneuver to control the "problem"'s outcome. In fact, it'll make you look downright competent, and like a good employee. When you go into a negotiation with a stacked deck like that, you go in with control, you maintain control, and you exit in control.
I might not be good at everything I think I'm good at, but I will punch myself in the throat if the strategy I've outlined fails. You've done a good job of identifying and outlining the problem, so I feel confident in what I'm saying. Create a problem, control the outcome.
Credentials: I'm doing work that usually requires 6 years of specialized education with 2 years of irrelevant education and no experience. If I know how to do one thing in this world well, it's negotiate. Best of luck to you, if you have questions don't hesitate to ask.
