How can I organize my content using hierarchical taxonomy?
I had a similar implementation on a drupal 6 site using Hierarchical Select. The drupal 7 branch turned out to be working slightly differently when I first tried to make the move. 

There is also Simple hierarchical select which was probably developed because of these 'differences' between d6 and d7:


  "This module has been inspired by Hierarchical select but is much simpler (…)."


I haven't tried 'Simple hierarchical select' yet, but if I would re-build that d6 site again, I would probably go for 'Simple hierarchical select'.

Both let you generate a taxonomy tree with dependend parent-child-relationships. So instead of having one taxonomy brandand one one taxonomy model you would have a taxonomy tree taht would contain all the branches down to specific model.

In my case is was venues which then also contained the upper branches like cityand country.

PS: You could however also use entity references to build complex hierarchical content trees. You would then for example have a content type brand and a content type model referencing each other.  Nowadays I find this approach more convenient than dealing with taxonomy trees – but that's of course depending on personal preferences and/or content structure.