drupal dyanmically populating drown down lists

hey first off i'm sorry if this isn't clear, it's been a long day....

i was wondering if anyone would know the answer to this, i'm looking for a way to dynamically populate drop down lists in drupal based on the selected taxonomy terms (select parent to list child term)

a good example of what i'm looking for is the search on the autotrader website http://www.autotrader.co.uk/

when you select a make of car ie "Vauxhall" a list of all vauxhall models is generated in the "model" drop down list

i've tried various modules such as hierarchical select it does what i want but the problem with that is that it starts off as one drop down list then more appear as you go through the taxonomy terms, i want all drop downs to be alreday viewable..

i've attached an image which will hopefully explain it better