That's what I did. I'm not sure what I'm misunderstanding? They are plugged into my miner and I shoved my probe into the back of the plug where the contact is and grounded to the case. I got 12V with it running and plugged in.
LEPA ATX12V/EPS12V CrossFire Ready 80 PLUS Gold Certified Power Supply G1200-MA
They're made by Enermax, the same PSU company that mine fails to mine with. It will flicker on for a second then sit in an error state as it thinks the sudden rise in power consumption from the miner is a surge/overload and will shut itself down.
Unfortunately you'll likely need a different PSU.