I put in a new HU worked fine. I go to put in old one doesnt work but the light is blinking on it. I take the black wire (grounding), and I touch some metal and it sparks, so there is power runnning. I go to put new HU in again, and that stopped working also. I'm thinking maybe a fuse?? Any Suggestions?
Check the fuses, on the radio and in your car. I bet they are blown. I am also going to guess your old stereo has a protection mode that has triped and thats why it wouldn't work when you put it back in.