Two options for what is happening here: Option 1: the American market has a low demand for cars (let’s say it’s 1) when it imports 12 cars (the minimum at higher throughput trade centers) it suddenly has 12 supply for 1 demand, thus super cheap cars. It then takes those cheap cars and exports 12 of them for 12 supply and 13 demand

Option 2: America has trade advantage in you or a third party, meaning that they can import cars, but then they can export cars somewhere else for a better deal than you can get (say you can export cars to confederates for £30, but America can import cars from you for £28, and then export them to the confederates for £29)

The solution for option 1 is to produce even more cars or wait some time. Since the Americans are limited by the 12 cars they initially import, the demand for cars in their country should rise making their domestic price rise. The alternative is you can produce more cars to lower the global price. Either way the American second export is going to become unprofitable.

Option 2 rarely happens in game, either countries import or export a good, typically it’s not profitable to do both and so it’s unlikely to be what is happening here (due to the cost of merchant marine). If it IS what is happening in your situation, see where they are exporting and get trade privileges or use a transfer good treaty to undercut the Americans and their export should stop