Depends on the carrier. ISO has a PAP trust endorsement but it is not a true AI endorsement and, frankly, is of limited value. I'm assuming the trust is the owner of the vehicle?
The first question is why does the trust need to be listed as an additional insured? Does the trust own the car? If so, why?
A lot of carriers will not write trusts, some will. If they own the car, then the trust should be the primary insured, not an additional insured, but of course, even more carriers steer clear of this type of thing. If the trust doesn't own the car, then why do they need to be listed as an additional insured.
Most of these situations come up when someone is trying to skirt some other (usually tax) rules, so carriers tend to realize its not a good game for them to play. Of course, lawyers don't think of this when they want to through everything in the trust.