It should be in the trust under your facts. As a minor, your son cannot own/inherit the car so the trust would own it or you could as a parent on his behalf. If this is a living trust, there are often issues with transferring cars into the trust (at least on the actual title document). If the car is part of her estate and the trustee is leaving it uninsured that is a big issue - what if someone takes it and causes a car accident? What if a tree falls on it? You need to hire an attorney to effieciently write a letter to the trustee to advise them to take care of insuring/transferring/moving the car and following the terms of the will. I doubt it will be worth the money to go to court over it, but a stern letter might get the job done.
This is not legal advice nor intended to create an attorney-client relationship.
You say the car was "willed" to your son. Is there a specific provision in your mother's will? If so, the car is not a trust asset but is an asset of the probate estate. Under Nevada law, if the car is the only estate asset and is valued at $20,000 or less, the title can be changed by affidavit at DMV (http://www.dmvnv.com/pdfforms/vp024.pdf).. Since your son is a minor, the affidavit would need to be signed by an adult, and DMV may require that it be a court-appointed guardian. If your mother's will leaves everything to her trust, then the affidavit would be signed by the trustee, which would be better. Depending on the wording of the trust, the trustee may be authorized to let you take title for your son, but the trustee may also be authorized to sell the car and spend the money for your son. If the trustee is not doing the job, the recommendation of Mr. Zelinger to have an attorney send a "stern letter" makes sense.
The real answer to your question depends on the wording of the will and trust documents, and you should consult an attorney to review those documents to determine the best course of action.
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