Your understand the law well.
But there is one exception, in that Coca-Cola is a “famous” trademark that is subject to “dilution” of its immense value by use on unrelated goods.
Same for Rolex, Smith and Wesson, Chanel, and other zillion-dollar brands that are recognized by all consumers, not just the consumers of their products.
It seems like unfair corporate welfare, unless you consider that these brands can and often do earn major income by licensing their brand for a wide range of goods.
So, if the vehicle folks or their predecessors from whom they acquired the rights started using Jeep first (and continuously, which is a potential concern) then they may have rights against a saloon. Just as the Xerox saloon wouldn’t last long.
While I’m pontificating, it annoys the amateurs that one has no legal right to use one’s own name as a trademark if it infringes the rights of another trademark owner. This often crops up when someone tries to open a coffee shop using a play on the “Starbucks”. Lesson: if the point of your trademark is to make someone think of another trademark, you’re probably infringing.
The very fame of Coca-Cola would tend to forbid using “Coke” for something that is not related, then. Kind of makes sense. A less famous name like, say, Domino (sugar and pizza, separately) could be reused for a computer and they would be unlikely to care.
With Duck Dynasty taking off in spades, I wonder if redneck/camo-theme marketing concepts, especially newer ones, are going to come under fire. I think the Duck Commander folks might be less concerned because they have a life (as long as it is not a blatant rip), but A&E could be more prissy about their spin on it.