Mercury Topaz Back Up Light Bulb Size

The Mercury Topaz back up light utilizes 3157, P21W, and 1156 bulb sizes depending on the specific version of the vehicle. Available data for bulb specifications covers the range from 1984 to 1994.

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
1994	3157
1984-1989	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Dual-filament wedge bulb (T25). 12V, 27W/7W.	3157
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156

Other Locations

Additional Notes

The 1156 bulb is the most frequently specified back up light bulb for the Mercury Topaz, appearing across the 1984-1989 model years.
The 1156 operates on a single filament and uses a BA15s bayonet base, running at 12V with a rated output of 21W.
The BA15s bayonet base features a single contact pin, which physically distinguishes it from dual-contact bayonet bases and prevents cross-installation with incompatible bulb types.
Both bulb types listed in the Mercury Topaz specifications conform to ANSI standards recognized in the United States and Canada.
The 1994 Mercury Topaz uses the 3157 bulb for the back up light, which differs from the 1156 used in earlier model years.
The 3157 is a dual-filament T25 wedge bulb rated at 12V with a 27W/7W output, where the two wattage values correspond to its high and low filaments respectively.
The wedge base of the 3157 and the BA15s bayonet base of the 1156 are physically incompatible, meaning the two bulb types are not interchangeable.
The shift from a single-filament bayonet bulb to a dual-filament wedge bulb between earlier model years and 1994 reflects a change in socket type that requires attention when sourcing a replacement.