Political affiliation is definitely part of social standing and the degree to which such signaling impacts how people talk, dress, and what they buy is very much fashion.
As you venture out into the world you will also find trucks that are clear political statements. You might be right that an individual cannot own a truck for both fashion and political reasons (although I'm not sure I agree), but just because some people use trucks for political statements does not mean all trucks are political statements. Some trucks can be used for fashion without crossing those who use them political statements.
I happen to think that most modern trucks look incredibly stupid, but that's because their design language isn't intended for me. It's intended for people who have different fashion tastes, tastes that approximate political positions that I don't hold. In other words: the truck itself isn't a direct political statement, ordinarily; it's an efficient means for signaling politics through fashion.
As a Canadian living in the heart of supply management (i.e. state ownership of the means of production) country, where supplies are provided by the so named and literal Co-op (i.e. community ownership of the means of production), trucks are very popular. In part because they are useful tools in carrying out the business within that socialist system, but also because outsiders emulate to try and convey similar social standing.
If there is a political statement to be found in there, which may be a stretch, to me it would be that trucks = socialism. But when I look beyond my bubble, I'm left with the impression that trucks are, if anything, especially stateside, a statement at the compete opposite end of the spectrum.
What approximate political position do you believe is inbuilt into the design of modern trucks?
If people decide to broadcast their opinions about the world using cars, they certainly can; but I don't think that kind of behavior can be called "fashion". Ideology, maybe.