The Scion xD back up light utilizes a 921 bulb size for illumination purposes. Available data covers the period from 2008 through 2014 for the specified vehicle components.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Scion xD uses the 921 bulb for the back up light across the full 2008-2014 model year range.
The 921 is an ANSI-standard wedge bulb in the T15 form factor, rated at 12V and 16W, and is a single-filament design commonly used in reverse and interior lighting applications.
The outer back up light on the Scion xD uses the 3057 bulb across the 2008-2014 model year range, with the 2014 application noted as valid only through July 2014.
The 3057 is an ANSI-standard dual-filament wedge bulb in the T25 form factor, rated at 12V with a high filament of 27W and a low filament of 7W, a configuration typically associated with combined stop and tail light functions.
The presence of a dual-filament bulb in the outer back up light position indicates that the outer assembly serves more than one lighting function, with the 3057 handling both a brighter and a dimmer output depending on the circuit activated.
The 921 and 3057 are not interchangeable. They differ in filament count, wattage, and physical form factor, with the T15 and T25 designations reflecting distinct wedge base sizes.
Both bulb types conform to ANSI standards applicable in the United States and Canada, meaning they meet the same regulatory specifications in both markets.