My first priority would be clothing. Thermal regulation is job one. If I have to step out of that vehicle en route, how will I fare? At the very least, I'd want a jacket with something like GoreTex shell on the outside and fleece lining. In the side pockets, I'd have a wind-blocking fleece hat and gloves. I might roll up some light weight rain/wind proof pants and put 'em in my brief case. This time of year, most of this should be pretty unobtrusive unless it's 84F and sunny out.

My second would be map(s). Merely knowing where you are can get you out of a jam. Not knowing can get you into one. Trust me on that last one.

Of course I'd want my normal EDC stuff:
knife
multi-tool
whistle (on lanyard)
compass (also on lanyard)
mini-squeeze light (also on lanyard)
ignition source (lighter, sparker, or whatever)
cordage (braided into a key fob)
P-38 can opener (on key chain)
bandana
cell phone
pen
paper
wallet
chapstick
sunglasses

I carry things like the above everyday, and no one comments except perhaps if I actually use my knife or multi-tool.

In my bags, a fanny pack or extra shaving kit as mentioned above (excellent suggestion by the way) with some gear would be good. I think that's been covered well by posts of others.