Unless your dealership is also a truck stop, I would be very skeptical of Sunday hours.
If there was a determination to extend hours, I would look more at evenings or even 24 hours, depending on your location.
In certain areas customers would take advantage of being able to have service performed when they are not using their car during the normal work day. It would be nice to drop off a car after 5pm for a timing belt and pick it up at 7am to go to work the next day.
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Tom Ham
AutomotiveManagementNetwork.com