It isn't completely clear if you also have a Data Access Layer because you mention "so that if you ever need to update the way services are done or where the data is stored, you do not need to touch the Customer\Vehicle module". The DAL would take care of handling the retrieving and storing the data, where ever that may be.
But of course it could be that Vehicle must be updated and it would be most convenient if updating those rules needed to be done in one place. You could simply create a Customer and Vehicle BLL with those rules. Then you just add a CustomerVehicleService that uses the Customer and Vehicle. There isn't a rule you can't.
Maybe this link could also help you: