My base vehicle kit pretty much is the same year round, except that I do add an extra duffle with a winter clothing supplement, such as a hooded ski jacket, insulated bibs, fleece hoodie, Sorel boots, gloves, hat, a couple extra pairs of heavy socks, some extra chemical warmers, including boot warmers, things like that. I tend to have an extra blanket or two beyond the one in my regular kit. In winter, I carry a snow shovel, in addition to the folding GI style shovel in the regular kit. There's probably other stuff I carry in winter that I don't in summer, like windshield de-ice/washer fluid. This was an interesting question! I don't really know everything that's back there...

I do have all weather clothing items in the base bag, and enough to get thru the cold, but, always figured it wouldn't hurt to have an extra set of cold gear in winter. Sometimes it has been used by a passenger, since I tend to have weather appropriate clothing on anyway, but, I've had folks in the truck that aren't as prepared.

I guess in the summer, I do tend to carry more water, because I usually have an extra case of bottled water in the back. I think my base kit's pretty good all weather, but, I reinforce with the season and on longer trips. (Probably need to be more conscious about it, rather than just doing it when I think of it.)