You need separate table occurrences of the same table so that you can define one relationship for your conditional value list and a different relationship for matching to a specific model and year.
In Manage | Database | Relationships, select Vehicle Volumes by clicking it. Then click the duplicate button (has two green plus signs) to make a new occurrence for this table. You haven't duplicated your table, just added a second label that refers to the same table. You can double click this new occurrence box if you want to rename it to something more descriptive than "Vehicle Volumes 2".
Here's a tutorial on Table Occurrences that you may find to be helpful in understanding how this works and how you can use this concept to get more from your FileMaker database: Tutorial: What are Table Occurrences?