If it is done as a statement of faith or for some other just purpose, hanging rosary beads on the rear-view mirror would not violate canon law’s requirement that sacramentals be treated with due reverence:
Sacred objects, set aside for divine worship by dedication or blessing, are to be treated with reverence. They are not to be made over to secular or inappropriate use, even though they may belong to private persons (canon 1171, Code of Canon Law).
However, beads draped from a rear-view mirror may impair one’s view of the road through the windshield. If the chain is heavy enough, over time it can also drag down the mirror, impairing the mirror’s usefulness for checking behind your car. If the beads constitute a driving hazard, on that ground one should not hang them on the rear-view mirror.
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