Where to find SATA Power?

I want to add a USB3 PCIe adapter card to my Mac Pro (2010), but it requires to be plugged into a SATA power line.

1. My first attempt was to plug a SATA power extension into the SATA power connector for disk sled 4. That didn't work. I think that connector isn't active unless a disk is on the sled. I can see a pressure switch for that purpose.
2. My second guess was to connect into the second optical disk SATA power, but I would need to drill into the optical disk sled to pass the wires through.

Does anyone know a better place to connect to SATA power?