I would be chopping out all three (red, yellow, black) and running new cables for all three. The problem is more than likely somewhere that the previous owner had run a dodgy cable that's going to ground. The black is ground and of the red and yellow, one is ignition 12v+ and one is memory +. If you trace them one at a time you should be able to replace pretty easily. Maybe bump up the cable size also. The ignition wire will go to a bus/fuse block of some kind that runs of ignition, and the memory will go to a permanent supply bus/fuse block. Same place your clock power will come from. Sorry if that's not much help. It's hard to diagnose these problems over the internet haha.