If you are talking about the old ISO hired and nonowned auto endorsement, there can be a problem under the circumstances you describe.

The hired auto liability coverage applies to bodily injury or property damage arising out of the maintenance or use of hired autos in the business of the named insured by the named insured or its employees. So, under your description, the vehicle is not used by the named insured or its employee but by a subcontractor. So, the insuring agreement for hired autos is not met.

As for the nonowned auto liability, the insurance applies to bodily injury or property damage arising out of the use of any nonowned auto in the business of the named insured by any person other than the named insured. At first glance that would seem to fit your scenario, but look at the definition of a nonowned auto. The definition states that a nonowned auto is an auto not owned by the named insured and not hired or used under contract in behalf of the named insured. So, if the subcontractor has a contract with the named insured to work on the premises, the definition of nonowned auto is not met and the coverage is not there.

Now, the ISO endorsement is an old one and not used by ISO anymore and your endorsement may be worded differently, so check the wording on your endorsement. The wording on your endorsement may allow coverage but there would be a problem with the ISO endorsement.