Usually with situation in an older vehicle, I redo the harness so I can see if the issue can't be found because it was a rubbed through situation that melted away the evidence and cause.
But I also make sure once new harness is in place, before I make it pretty and put loom on or secure it anywhere, I do small tests of powering it to see if anything begins overheating and then I start eliminating things at the end of each circuit of harness
If you can't keep a meter with a beep on the suspect, having a friend feel for heat is the next best thing haha
But I go over it a lot, since dead customers can't pay. It's a tedious process and you may find more than one possible cause:
Previous owner hacked into this wire for power to radio
Previous owner hacked that wire for power to their cb
Previous owner hacked this wire for ground to their gauge