Hi Martin,
You don't necessarily need a script for this as you just want the values presented in the second field to be "conditional" on the values in the first field. Search FileMaker Pro 7's online help for the phrase "value list". The first entry is a two page article that goes into this; you're looking for the first bullet point presented:
From a field in the current file or another file. This method provides flexibility because the value list can dynamically update itself based on the current values in a field. If the field is in a related table, you have the option of showing all the values in the field, or only the related values.
In your case you'd be interested in the "related values", specifically all the models (Focus, F-150, etc.) related to the manufacturer (Ford). The details of how to do this are interspersed over the two page article and will require you to set up two tables, one of manufacturers and one of models, with a relationship between them.