You didn't say "Good" or "skilled" mechanic..lol. can you post a picture of the panel area? Those wires on top of the battery provide electrical power to various systems. The exposed metal wires are fuses. Those get melted if they have poor connections. Also the ground cable under battery tray gets very corroded and that may be causing a bad ground connection between the engine, body, electrical system maybe that's part of all your problems. That sensor needs a good ground through the system. If you take the battery out and the tray underneath the ground connections are there and usually in poor shape. My guess is that only a portion of your battery voltage is making its way into the electrical system. Can you post a picture of the top of the battery area?