Hi Ben Otto, just set the "model" and "build" as an Auto Enter field and check Data and leave the value field blank. What will happen is everytime you change the value of your "car brand", your "model" and "build" field will be emptied. Say you have Ford -> Mustang -> 1980 and change Ford to Toyota, you will have Toyota -> "model (empty) -> "year (empty)".
Hi Ben, I just reviewed my answer above and realized that it will not work, my apology.
So back in the topic, you do have to set a script that will set your downstream fields (model & build) to blank as soon as you change your main field (car brand). Say you have a script called "Set Model and Build to blank". This script will have a Set Field script step like below:
Set Field [Model; ""]
Set Field [Buid; ""]
You will then set this script as a Script Trigger and put it on OnObjectModify event of your main field (car brand). That will do it Bob, hope this helps and again I'm sorry for my first answer above.