Thank you for your post.
If a car-brand does not appear in the the pop-up, you would need to add the car and brand in the related table. If you are using FileMaker Pro 10, you could use a script trigger that checks for a "Other" option, and if true, switch you to the related table to enter the information.
If you are using FileMaker Pro 9 and earlier, you could create an "Other" button that also executes the same script.
Let me know if you need help in setting this up.