Mercury Monarch front side marker bulbs utilize 194 or 1157NA bulb sizes, while rear side marker bulbs require 1157 bulb size. The replacement chart covers 1977 through 1980 production years with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercury Monarch uses three distinct bulb types across its front and rear side marker positions: the 194, the 1157NA, and the 1157.
The 194 is a miniature wedge bulb in the T10 form factor, rated at 12V and 3.78W, and conforms to the ANSI standard recognized in both the United States and Canada.
The 1157 is a dual-filament bulb with a BAY15d bayonet base, rated at 12V with a 21W/5W filament configuration, also conforming to the same ANSI standard.
The 1157NA shares the same base type, voltage, and wattage ratings as the 1157 but is distinguished by its natural amber glass envelope, which produces a colored output without the use of an external lens filter.
The front side marker position on the Mercury Monarch used the 194 bulb for the 1978-1980 model years.
The 1977 Mercury Monarch front side marker position used the 1157NA rather than the 194, making it incompatible with the fitment used in the three subsequent model years despite occupying the same position.
The rear side marker position used the 1157 bulb across all four model years covered, from 1977 through 1980.
Although the 1157 and 1157NA share the BAY15d bayonet base and identical electrical ratings, substituting one for the other affects the color output of the lamp, since the 1157NA produces amber light through its tinted glass while the 1157 relies on the surrounding lens for any color filtering.
The single-filament 194 wedge bulb and the dual-filament 1157-series bayonet bulbs are physically and electrically incompatible with one another, as they differ in base type, wattage, and filament count.