Mercury Custom Back Up Light Bulb Size

The Mercury Custom back up light uses bulb size 1141 or 1129 depending on the specific version of the vehicle. Data for the bulb sizes covers the range from 1952 to 1956.

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
1956	1141
1952-1955	1129

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1141
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. Variant of 1157. 12V, 21/5W.	1129

Other Locations

Additional Notes

The Mercury Custom backup light specifications cover model years 1952-1956, with two distinct bulb types used across that span.
The 1141 bulb is an ANSI-standard single-filament type rated at 12V and 21W, fitted with a BA15s bayonet base. The BA15s designation indicates a single-contact base with a 15mm collar, which physically prevents substitution with dual-contact alternatives.
The 1129 bulb is an ANSI-standard dual-filament type rated at 12V with a 21/5W output split across its two filaments. It uses a BAY15d bayonet base, which features offset pins to ensure correct orientation during installation.
The 1129 is noted as a variant of the 1157 bulb. While the two share the BAY15d base and a similar wattage profile, they are not identical, and substitution between them warrants verification of exact electrical and physical compatibility for a given application.
The Mercury Custom (1956) uses the 1141 bulb for the backup light, while the 1952-1955 model years use the 1129.
Because the 1141 and 1129 use different base types, BA15s and BAY15d respectively, the two bulbs are not interchangeable with each other regardless of their similar wattage ratings.
The shift from the dual-filament 1129 to the single-filament 1141 between the 1955 and 1956 model years reflects a change in the socket and wiring configuration, meaning a 1956 socket is designed only for single-contact bulbs.