Ahhh, one of these again
Good place to start might be with the ground points, they're usually the main culprit. I had random electrical issues after I had the bulkhead/firewall patched (area was rotten through). Turns out the mechanics cut out the old rot, welded in a new plate, and covered it with rust inhibitor paint, but also painted over the ground point in the firewall, so the grounding was screwed.
Anyway, start with the ground points as they're the easiest, and if still nothing, may be worth tracing the electrical failures to one point using the electrical schematic.
Good luck!
Victor