A lot of Preventative Maintenance depends upon the LOCAL interpretation of the FAA. I’m an A&P/IA and a customer was REMOVING brake assemblies for a Cleveland brake STC. I wasn’t there when the FAA guy peeked into the hangar and cited the individual for performing work not authorized as a pilot without an A & P. So I had to present my case that since he was only REMOVING , not installing the brake assemblies, that he shouldn’t be cited. That cleared the issue but you have to be careful what you are doing. As far as electrical wiring repairs, I doubt that most FAA folks would consider that Preventative Maintenance. If you are installing LED’s in lieu of incandescent bulbs, you MAY need an STC and a 337. And I can’t tell you how many aircraft I have worked on where the owner added AUTOMOTIVE brake fluid , NOT -5606! I highly recommend that owners work closely with an A & P or IA to stay out of trouble, and stick to those tasks that they are qualified to do.