That's actually completely untrue. There are buttons on the trim fixtures tab which allow you to replicate the model fixtures tab to the current trim, or to allow you to copy the current trim to all models. I'll post a pic, if you need help with this.
This is the Model Fixtures Tab. It is the first tab you encounter for fixtures when building a new car, as indicated by the box at the bottom of the screen. Note there are only Undo/Redo buttons on this screen. This tab defines the template for all new Trims created under this Model.
**NOTE - Changes made on this tab will NOT apply to existing trims unless they are changed on the tab described below.
Pictured below is the Trim Fixtures Tab, as indicated by the box at the bottom of the screen. Notice it has two buttons not present on the previously shown Model Fixtures Tab. Any changes made here and not copied to the template will only appear on this Trim.
The left-hand button allows you to copy the Model Template from the first tab to this tab. It is essentially a fixture reset to the template design.
The second button allows you to copy what is seen on this screen to the Model Template for all Trims. This can be used to apply changed made on the Model Fixtures Tab to the current Trim.