I get 11.52v across the source by itself, but when connected to vin and gnd it drops to 1.5v
...that got me thinking though, and it looks like I've resolved the problem.
I was using one of the smaller fan power connectors on the supply for the more convenient smaller gauge connector inputs. It supplied 12v unloaded, but I think it had some sort of current limiting circuitry built in. I switched inputs over to one of the main 12v lines that you would use for your disk drives and everything came up green