Need technical information...StreetPilot C550

So I'm working on an old StreetPilot C550 for a friend. The battery was shot, so it only worked when plugged in (i.e. you plug it in, it turns on. you unplug it, it shuts off immediately).

After installing the new battery, I come to find out that the power button doesn't work, so I can only turn it on by connecting a power source (12V or USB). Problem is, I can't turn it off other than hitting the reset button under the faceplate. It's possible I may have lost some piece that goes between the external button and the PCB, but I don't think so. Is there a schematic or other diagram anywhere that will show me if I'm missing something? If not, then the button is bad, I think. Any help would be appreciated.

Loopie