How often should commercial vehicles be inspected?

I have checked with manufacturers, with my service provider.

Perhaps I should be asking how to get a business to actually do vehicle inspections

I have done safety inspections and found vehicles with zero oil, vehicles 20K overdue for a service.

No cargo barriers in vehicles (with heavy equipment in the back)
Headlights and other lights not working.

I have told the owner that we could have a vehicle seize an engine while on the freeway and that would be a huge hazard (to which he answered"That has already happened to us")

I am tired of continually asking for this to be done with no result (yes I have put it in writing)

What would the more experienced Safety Managers do?