This first occurred to me years ago when watching "Murphy's Romance". Sally Fields was a struggling single mother trying to run her farm and only had an old truck. Then, of course, "Mr. Majestyk" with the famous chase scene in the old truck. Even Clint Eastwood in "Gran Torino" mostly drove around in an old truck.
It's always used to convey the inherent virtue of the protagonist and to position the protagonist as the underdog since the antagonist will usually be driving something flashy like a sports car or a Mercedes sedan. Guess we could probably call that the "Evil Merc".
Used to drive my wife nuts, when I had a wife, when The Virtuous Truck would appear and I would call it out.
I suppose it's sort of like the good guy wearing the white hat, probably the oldest trope of all.
Anyway, first time here, so be gentle with me. Funny, interesting site!