Possible Causes:
* Faulty Ignition Switch: While you've unplugged it, the switch itself might be internally shorted, constantly providing power even when it shouldn't. This is a less likely scenario given your description, but possible.
* Short Circuit in Wiring Harness: This is a highly probable cause. A wire might be chafing against metal, causing a short to ground, and constantly feeding power to the accessories. This is often caused by rodent damage or wear and tear on the wiring over time. This needs to be investigated thoroughly.
* Faulty Body Control Module (BCM): The BCM is a computer that controls many electrical functions. A malfunctioning BCM could be the culprit, constantly sending power to the accessories.
* Faulty Alternator: While less likely to cause this *specifically*, a failing alternator can sometimes overcharge the system, leading to strange electrical behavior. A parasitic draw could further strain the system, keeping things powered on.
* Parasitic Draw: This is the most likely explanation. A constantly active circuit is draining the battery, even with the key off. This is often due to a faulty component that's not turning off.
* Incorrectly wired aftermarket accessories: If you've added any aftermarket accessories (stereo, lights, etc.), it's possible there's a wiring fault.
Troubleshooting Steps (Proceed with caution – always disconnect the negative battery terminal before working on the electrical system):
1. Disconnect the Battery: Completely disconnect the negative (-) battery terminal. This is crucial for safety before any further work.
2. Check the Battery and Alternator: Use a multimeter to check the battery voltage. It should be around 12.6V when fully charged. Also, have the alternator tested at an auto parts store to make sure it's charging correctly and not overcharging.
3. Check for Parasitic Draw: With the negative terminal disconnected, reconnect it using a multimeter in series. The multimeter should read the current draw (amps). A normal parasitic draw is usually less than 50 milliamps (0.05 amps). If it's significantly higher, there's a parasitic draw. This requires systematically disconnecting fuses one by one until the draw disappears; that will isolate the circuit with the problem.
4. Inspect the Wiring Harness: Carefully examine the wiring harness, particularly in areas known for chafing (around the steering column, under the dash, and near the engine). Look for damaged, frayed, or melted wires. Pay close attention to the wiring near the ignition switch.
5. Fuse Check: Carefully inspect all fuses and relays in the fuse box. Look for any blown fuses or melted relays. Replace any faulty ones with the correct rating.
6. Professional Help: If you're not comfortable with electrical troubleshooting, take it to a qualified mechanic or auto electrician. This issue is potentially complex and requires expertise to diagnose and repair safely.
Safety First: Working with car electrical systems can be dangerous. If you are not experienced with automotive electrical systems, it's best to leave the diagnostics and repairs to a professional. Incorrectly diagnosing and fixing this could lead to further damage or even fire.
