Missing display fields in a lookup field

Hi Everyone,
I have a lookup field that has a filter set based on a previous lookups selection. Similar to the country and state scenario.
My scenario is a vehicle make, model and variant scenario though.
I have a form for each - one for make, one for model, one for variant.
Make just contains a single line for vehicle make
Model contains a lookup to the make field and a single line for model
Variant contains a lookup to make, model and a single line for variant but also a dropdown for year from, and year to.
I have another form that allows you to select a make, model and variant. On the variant selection I would like it to display [Variant - Year from - Year to]. However, the year dropdowns do not appear in the user defined fields of display dropdown.

Any ideas anyone?

Thanks