That's the fuseable link and main power supply to the inside of the car. Originally there would have been a wire going to the bulkhead connector that had the other half of that white connector. If the wiring diagram that Mattax posted is accurate, that main power wire goes into socket 16 (one of the red wires highlighted in the diagram). If you follow the wire back, you can see the connector for the fuseable link section, and then the starter relay connection end.