Mercury Monarch Back Up Light Bulb Size

The Mercury Monarch back up light uses 1156 and P21W bulb sizes according to the provided technical data. Information for the back up light covers the 1975 to 1980 range.

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
1975-1980	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 Mercury Monarch uses the 1156 bulb for the back up light across all listed model years, spanning 1975-1980.
The 1156 is a single-filament bulb, meaning it serves one lighting function only, which is consistent with its application in a dedicated back up light circuit.
The 1156 operates at 12 volts and 21 watts, placing it within the standard range for incandescent automotive reverse lighting.
The 1156 is built on a BA15s bayonet base, a single-contact fitting where the "s" denotes a single pin offset to one side of the base.
The 1156 carries ANSI standardization, indicating it conforms to a defined specification recognized across the United States and Canada.
The BA15s base of the 1156 is not interchangeable with the BA15d base found on dual-contact bulbs such as the 1157, despite the two sharing a similar physical profile.