I have a similar issue. I bought a "parts book" from the same guy I got my G3 12" for a working CD drive. While in there I upgraded the HD to 120GB. Now I get BLACK SCREEN. I double checked the power (blue/white) on top and the ribbon cable on the bottom but can't find anything loose. I totally forgot how to do PRAM. Funny thing. When I started this project the parts book came on fine even without a ram chip. I broke the thumb pad ribbon cable connection clamp on it so I can't even try or the screen in the parts book. I think I may have pain=ed myself ion a corner this time. So much for DIY. I'm bookmarking this thread to follow it for ideas.
No reason to disconnect the DC-In cable. Nothing we are doing requires it disconnected and it is a fragile and vital piece of a "working machine."