Use RockAuto.com's "Part Number Search" tab to identify that black box on the firewall or some other mystery part. Even a partial original equipment or aftermarket part number is often enough. Just use "*" for any missing or illegible characters. For example, a search for *339298* will find any part numbers that contain 339298.
Look for the Part Number Search tab at the top of the RockAuto.com catalog or use this link to go directly there.