I has Problem like that with a 12 volt accessory panel in my coach.. First time I noticed the 12 volt compressor freezer beside me was not working. Searched, no 12 volt, went to the distribution panel, no 12 volt, pulled the cord off it that feeds it (Anderson power poles) good 12 volt on the volt meter..... BAD with a test light (A test light is a fantastic diagnostic tool) went to the other end of the wire,, ONE CONTINUOUS LENGTH OF WIRE, No brakes No junctions (Measured at the wire side of the fuse) good, Bright light,,, Found the problem about 12-18 inches down the wire, it had a bulge,, Turned out be be all copper oxide in that spot.. Did the slice and solder and seal bit and it was good.. Till just recently.. and then the junction between fuse holder lead and wire came apart when I pulled on it during testing.. Did the solder and seal bit again,,, And it's back to all good.
Solder and seal is very effective.
