Just for fun I pulled out my multimeter and measured the current at the pins on the MB. It is 12v. I had the new fan wired directly to it just like the original setup.
I switched that connection to be inline with the main power to the HDD using the method above. Now it is running at 7v, slower, and just as quiet as when it was new. I checked the heat and it appears to still be cooling OK.
I'm pleased with the results. Again thanks for the help and education.