Michigan's concealed weapon prohibition provides an exception for property owners carrying (open or concealed) loaded pistols in vehicles. The statute does not provide an exemption to others who have permission from the property owner to carry a loaded pistol in a vehicle on their property.
I remember seeing a DNR prohibition on travelling in vehicles with loaded weapons. The last time I read it, the text had been modified to ban loaded long guns in vehicles while allowing for the legal carry of pistols in accordance with the law.
THE MICHIGAN PENAL CODE (EXCERPT)
Act 328 of 1931
750.227 Concealed weapons; carrying; penalty.
(2) A person shall not carry a pistol concealed on or about his or her person, or, whether concealed or otherwise, in a vehicle operated or occupied by the person, except in his or her dwelling house, place of business, or on other land possessed by the person, without a license to carry the pistol as provided by law and if licensed, shall not carry the pistol in a place or manner inconsistent with any restrictions upon such license.
(3) A person who violates this section is guilty of a felony, punishable by imprisonment for not more than 5 years, or by a fine of not more than $2,500.00.