It doesn't just help the domestic brands, it helps any truck built in the US even the Tacoma. When the Ranger left the market, the Tacoma had no real rival and sales sky rocketed. The chicken tax was actually protecting the Tacoma because other brands would have to build a factory here to get over the 25% tariff. They eventually did, but that did not drive down the price of the Tacoma.
Also, just because there is a 25% tariff on truck imports doesn't mean there is a 25% greater mark up on US built trucks. That is not how it works. There is still stiff competition between GM, Ford, Ram, Nissan, and Toyota.
