In my experience, it's the alternator 80% of the time. Of the rest, it's worn, frayed and failing wiring in the engine compartment, or a regulator failure, which is rather rare. Like I say, the alternator and regulator wiring are supposed to be checked at 100 hour/annual intervals. I've found badly worn wiring that previous mechanics (or cheap owners) just kept taping up, or it had to have been ignored altogether. A regulator loose on the firewall has poor grounding and will cause all sort of trouble, Found that, too. An overvolt sensor with loose or corroded connections is another source, and that's in wiring under the panel that often doesn't get checked as per inspection requirements. Drive belt failures are rare, but if they fail it's because they weren't replaced when they got old and hard and started cracking and delaminating. On the Lycoming the prop has to come off to replace the belt, so it gets deferred too often. In aviation maintenance we have the Dirty Dozen of Human Factors that contribute to mistakes and resultant aircraft accidents or incidents: 1. Lack of communication 2. Distraction 3. Lack of resources 4. Stress 5. Complacency 6. Lack of teamwork 7. Pressure 8. Lack of awareness 9. Lack of knowledge 10. Fatigue 11. Lack of assertiveness 12. Norms Of these, inadequate inspection can be blamed on at least Complacency and Norms. Norms are the habits the staff of a shop get into, and if a shop has the norm of doing an annual inspection in an hour, you're going to get a lot of wear and tear and incipient failures overlooked. If a customer puts Pressure on the shop to do it cheap, it gets shorted. Poor training leads to a Lack of Awareness and a Lack of Knowledge. Carrying a cellphone around in the shop is a horrible Distraction. There are lots of ways things can go wrong, and someone eventually pays a price of some sort.