Mazda Cosmo Back Up Light Bulb Size

The Mazda Cosmo back up light requires bulb size 1156 or P21W depending on the specific assembly installed in the vehicle. Data for the bulb sizes covers the range from 1976 to 1978.

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
1976-1978	1156

Bulb Specifications

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

Other Locations

Additional Notes

The 1156 is the sole bulb type listed for the backup light of the Mazda Cosmo across the 1976-1978 model years.
The 1156 is an ANSI-standardized bulb, meaning its physical and electrical specifications conform to a defined North American standard recognized in both the United States and Canada.
The bulb operates on a 12-volt electrical system and draws 21 watts, which is typical of incandescent single-filament automotive bulbs used for backup lighting in that era.
The 1156 uses a BA15s bayonet base, a single-contact fitting where the bulb is inserted and rotated to lock into place, as opposed to a screw or wedge base.
Because the 1156 is a single-filament bulb, it serves only one lighting function per socket, distinguishing it from dual-filament variants such as the 1157, which would not be a compatible substitute.