step one - put the factory head back in there and see what happnes - if it powers up, then either your wiring adaptor is bad, the harness for the aftermarket player is bad, or the head it'self is bad.....

that's the easiest way imo.

The other way to test it, b/c it seems you are good with a dmm.

Start at the factory plug, test for 12v+, next plug in the adaptor for the aftermaket harness - test for 12v, attach the aftermarket harness, test for 12V. Somewhere, you are going to lose 12v.

If you don't, then the problem is in the headunit. There are generally fuses in the harness for the aftermarket head, check those as well...