The reversegear signal input wires as stated in the installation booklet shown to hooked up to the reverse light, I did so. But the cable was not long enough to reach the head unit, so I cut it in the middle.

 

During installation (camera etc,) I put power to that wires to test my camera... can't see anything, but the screen shown the "reverse camera". So I blamed on the camera. Today, I just found out the rest of the cable has "fuse resistor" before it hooked up to the reverse light. Now I hook up the wires normally, but the screen didn't go to the "reverse camera" anymore when I put the trans in the R. I check the voltage, I got 3V. So I fed 12V into 3V. What should I do now?

That input expects 12V when in reverse.

Try opening the little housing where the fuse resistor is located. With the car in reverse with the reverse lights lit, measure the voltage on either side of the resistor. It should be 12V on both ends. If it's only 12V on one end, you may have blown the fuse resistor somehow. If that's the case, temporarily short across the 2 ends of the resistor and see if the HU detects the reverse signal. If it does, you just need a new resistor. You could also permanently jumper across it, but that may increase risk of fire hazard if there are any wiring problems.

Yep, the fuse resistor measured 12V that goes to the light, and only 2.2V at the other end of the resistor.

I measured resistance of the resistor is 1K ohm. So it's not disconnected, but not connected either. 2.2V goes all the way to the head unit now and head unit didn't recognize that signal as a reversesignal. But if I feed 12V into the reversegear input, the unit recognize and goes into camera mode, but so far only plain black. So still don't know if the camera good or not.

 

But thanks for your post, now, I have hope again.