I'm guessing you want to do this getting on/off the car, not during the trip. If so, you can do slide down on home screen to search, type "auto-lock." The link to the setting should show up if you scroll down. Once you've done that once, it should show up under "Siri Search Suggestions" the next time you swipe down to search (without actually typing this time, even if you clear the search field.) So from now on you can swipe down, tap, and change the setting. Not a single tap solution, but faster than manually doing it.
The app Launcher use to be able to take you to specific areas in Settings directly, but it looks like Apple's not allowing that anymore and Launcher's removed those options.
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You could ask Siri to do it ...
She will tell you that she can't do that, but will give you a button that takes you to that setting.
That's the quickest way I know, short of jailbreaking.
This is also good, but you also tell Siri to "Go to autolock settings," to avoid a step.