Best advice yet. Working on the electrical system, one should be able to a) either be able to read a wiring diagram, or b) be able to use a tester to ‘ring’ out the wires to determine which are ‘hot’, switched, or ground. Because without that knowledge, the wiring methods used to tap into that harness properly, i.e., using approved connectors, heat shrink tubing, etc. would also be called into play. I have seen many sketchy wiring jobs over the years from those not accustomed to these types of jobs.