From the Mi State Police website:
"1. If I do not have a CCW permit, may I transport my pistol in a motor vehicle?
A person is now permitted to transport a pistol for a lawful purpose if the owner or occupant of the vehicle is the registered owner of the firearm and the pistol is unloaded and in a closed case in the trunk of the vehicle. If the vehicle does not have a trunk, the pistol may be in the passenger compartment of the vehicle unloaded and inaccessible to the occupants of the vehicle.
..."
There are many other references (not singling out hunters) to transporting firearms in vehicles without trunks that read similar to:
"A hunter possessing a valid Michigan hunting license may transport a handgun to or from the hunting area in region 3 in
a motor vehicle, provided that the hunter is en route to or from the hunting area and
the handgun is unloaded, is in a wrapper or container, and is locked in the trunk of the motor vehicle. If the vehicle has no trunk, the hunter may carry the unloaded handgun in a wrapper or container in an area of the motor vehicle not readily accessible to the occupants of the vehicle...." (bold added for emphasis)
The short version of MI's Firearm laws is located
here.
Basically, keep the gun in a case (preferably locked) out of reach.