I had a brand new stereo stolen out of my Civic. When I installed the
new stereo, it wouldn't work either. Found out after that who ever broke
into my car must not have had wire cutters and pulled on the stereo
until the wires broke. I had to run new power and ground wires.
If you are only getting 4.3Vdc on the back-up lead, sounds like they may
have yanked the wire out of the harness to some degree. You should see
12Vdc on it as well. Try connecting a known 12Vdc source to all leads
that require 12Vdc.

B.