Montana is an open carry state as open carry is never mentioned in the montana code Annotated.
Concealed carry is generally prohibited as per MCA 45-8-3
However as per MCA 45-8-317 a concealed firearm may be carried outside the limits of a town, city, railroading, mining, or logging camp.
Restricted areas are in MCA 45-8-328.
Rest areas are not listed.
generally you can OC anywhere, no permit is required for open carry in a car either.
you can call the Missoula County sheriff, they'll be happy to answer any questions you may have.