Trade dress refers to the overall image. FCA is claiming the roxor shape is too close to their jeep image. For some clarification from legal summary:
In Two Pesos v. Taco Cabana (1992)‚ the United States Supreme Court held that trade dress can be inherently distinctive. When trade dress is inherently distinctive‚ it does not require secondary meaning to be eligible for trademark protection.
Trade dress that is inherently distinctive may be subject to trademark protection even if it has not acquired a secondary meaning. A mark may be trademarked if it is distinctive, per the Lanham Act. A mark is distinct if it is either (1) inherently distinctive, or (2) has acquired a secondary meaning. Secondary meaning refers to that situation where an aspect of a product that is not necessarily distinctive on its own has, either through successful marketing or some other mechanism, become associated in the public mind with that product. As a consequence of these definitions, a mark cannot be both inherently distinctive and have secondary meaning at the same time.
I’m not a lawyer but it appears clear that FCA is claiming the jeep shape is infringed upon in the USA by the Roxor, and that the jeep shape has achieved secondary meaning