You need to define two objects: Make and Model. Object Make should have multiple-allowed reference attribute Models of type Model to contain instances of all models for that make. The third object (where the user is supposed to select a make and model, lets call it Quote for the purpose of this example) should have two reference attributes: Make of type Make and Model of type Model with dropdown presentation option used for both attributes.
You also need to add a filter query to attribute Quote.Model that would look like this:
FIND Model WHERE Model IN Quote.Make.Models
The custom filter query can be entered in 'Filter' - 'Filter displayed items' on the right-hand side of the presentation properties dialog for attribute Quote.Model.
When the user makes a selection in Make dropdown, the system will automatically execute the filter query for attribute Model and re-display the form so that the Model dropdown will contain only those models that belong to the selected Make.
Please also see topic Constrained Value lists / Reference Attribute
Other necessary values can be taken from attributes of Make and Model selected by the user for Quote. You may also want to consider a form design with sequential (wizard) navigation style with two (or more) sections for the Quote form. The user could select Make and Model on the first section, then click Next button to proceed to the next section, that would show other Quote attributes filled with applicable values taken from the selected Make and Model attributes.
