1. Check the Ground Wire Connection: Ensure that the grounded bare wire is properly connected to a clean metal surface on the vehicle's chassis. Rust or paint can prevent a good electrical connection, so scrape away any obstructions to ensure proper grounding.
2. Examine the Wiring: Inspect the wiring harness for any damage, cuts, or loose connections that may have occurred during the speaker installation. Look for pinched or frayed wires, as these can also cause electrical issues.
3. Check the Speaker Connections: Ensure that the speaker wires are correctly connected to the speaker terminals. Refer to the vehicle's wiring diagram or the speaker manufacturer's instructions to ensure proper polarity.
4. Check the Head Unit/Radio: If the interior lights and radio are not working, the head unit or radio itself may be faulty. Try disconnecting and reconnecting the power and ground connections to the head unit. If this doesn't resolve the issue, the radio may need to be replaced.
5. Check the Fuse Box: While you mentioned that all the fuses are good, it's worth checking the fuse box again, paying close attention to any fuses related to the interior lighting or radio. Sometimes a fuse may appear visually intact but may have blown internally.
6. Check the Wiring Harness Connector: The wiring harness connector that connects the vehicle's electrical system to the speaker wires may be loose or damaged. Make sure it is properly connected and free from corrosion.
If you are still experiencing problems after checking these items, it's recommended to consult a professional auto electrician who can accurately diagnose and fix the issue.