I would have to second Thumper's take on the truck gun. Model 99s are in pretty high demand, and vehicles get cleaned out by thieves with depressing regularity. Guns in the cab also get jostled, stepped on and may get wet. If they're in plastic without silica gel or some type of vapor protectant, they'll rust up in short order.
A low-end milsurp or SKS may do the job without so much risk. Be careful about laws relating to guns in vehicles where you live. Around here, we used to see a lot of longarms in window racks. Didn't make sense to advertise, and a lot of them got stolen. If put out of view, however, may violate concealed carry laws.
Best bet may be an overhead rack above the sun visors, but I would still recommend an inexpensive gun for this duty.