Fiat X-1/9 Back Up Light Bulb Size

The Fiat X-1/9 back up light utilizes bulb sizes 1156, 1073, and P21W depending on the specific version. Data for the components covers the range from 1974 to 1980.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1980	1156
1979	1073
1974-1978	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. Lower wattage variant of 1156. 12V, 13W.	1073

Other Locations

Additional Notes

The 1156 bulb is the predominant back up light specification for the Fiat X-1/9, covering model years 1974-1978 and 1980.
The 1156 is an ANSI-standard single-filament bulb with a BA15s bayonet base, rated at 12V and 21W.
The 1979 Fiat X-1/9 uses the 1073 bulb rather than the 1156 found in surrounding model years.
The 1073 is also an ANSI-standard single-filament bulb with a BA15s bayonet base, rated at 12V but at a lower 13W output compared to the 1156.
Although both the 1156 and 1073 share the same BA15s bayonet base type, their wattage ratings differ significantly, at 21W and 13W respectively, which is a relevant fitment consideration.
The shared BA15s base means the two bulb types are physically interchangeable in terms of socket fit, but the difference in wattage output means they are not electrically equivalent substitutes.