Selected category updates sub category dropdown menu

Hi,
Been searching for a couple of days now and thought it's best to ask the experts as it's driving me bonkers.

I'm looking to build a parts catalogue (with no online ordering just yet but will possibly add it in the future) and don't seem to be able to work it out.

Basically I want to have a drop down menu to update and change to the next sub category when a parent category is chosen.

Not quite the perfect example as this uses 4 drop down menu's whereas I'd like to just use the one and it auto populate but this gives you an understanding roughly what I'm trying to achieve.
http://catalog.dbausa.com/

Really hope somebody has a simple solution that I've missed.

Thanks all.
Daniel