I've *never* had a power supply with a 12V Aux connector until now, and never even knew of its existence on any motherboards.

http://www.theregister.co.uk/content/3/11706.html

Note the date on the article. I knew about these, and I haven't purchased a computer since then, much less built one. You need to find the person that's supposed to keep you up to date on such things and give them a good talking to.