Are you sure the remote turn on lead is getting power? There should be a connection from the HU that supplies power when the HU is turned on. and a constant 12V feed of which you are probably seeing inside with your meter. One of the lines in your PB cable will be a "changer on" wire. Which wire that is? well i cant help you there. every HU is different.
It puzzles me that you dont have any lights. i would think, that you should at least, have the power light if the unit was at least trying to boot. even if the bootloader was corrupted, the power light should be on i would think.
I don't have any first hand experiance with these, but if i were the one working on it. that is what i would look for next. I would also look for cracked solder connections on the PCB near the PB connector and i would test the PB cable pin for pin to make sure the cable is good.
If all of that is good, then i would start checking to see if the IC's inside had power. thats a bit more involved, you would have to find spec sheets in each IC and figure out what pins are +VCC and -VCC etc.
9 times out of 10 it ends up being something mechanical, especially from what you described. unit worked fine, you disconnected the HU while the power was off. then reconnected the HU and dead box.
I would maybe suspect that the plug end has a bad wire in it at the HU side, or maybe that your radio has a bad solder connection inside where the PB cable plugs in etc...
Best of luck.
Zc