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Rest stops are marked by the blue bed icons on your minimap. Unfortunately despite having a bed literally in your truck you can't just pull over anywhere - it has to be one of the marked areas.

Some of the marked rest areas won't work either - e.g. those a bit off from your job's route inside another job site.

You'll know when you're in a valid area because as you come to stop there'll be a prompt to shut off your engine, and once its off there'll be a prompt to rest.