Lincoln Mark III Back Up Light Bulb Size

The Lincoln Mark III back up light utilizes 1156 and P21W bulb sizes according to the provided data. The information covers a range from 1968 through 1971.

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
1968-1971	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 Lincoln Mark III uses the 1156 bulb for the back up light across the 1968-1971 model years.
The 1156 is an ANSI-standardized bulb recognized in both the United States and Canada, meaning it conforms to a defined set of electrical and physical specifications shared across manufacturers.
The 1156 operates on a 12-volt system and draws 21 watts, which is consistent with standard automotive electrical systems of that era.
The bulb is a single-filament type, meaning it serves one lighting function only, as opposed to dual-filament bulbs that handle two functions such as braking and running light from a single unit.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a bayonet cap with a 15mm base width and a single contact pin, which determines physical fitment into the socket.
The BA15s single-pin base of the 1156 is not interchangeable with the BA15d base found on dual-filament bulbs such as the 1157, despite the similar base width, as the pin configuration differs.