Mercury Marquis high beam headlight bulbs use H4651 or H5001 size, while low beam headlight bulbs use H4656, H4652, or H5006 size. The replacement chart covers 1967 to 1986 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.
All headlight bulb sizes listed for the Mercury Marquis across both high beam and low beam positions are sealed beam units, meaning the entire lens-reflector-filament assembly is replaced as one piece rather than swapping an internal bulb.
Four distinct sealed beam types appear across the specifications: the H4651 and H4656 in a 4x6-inch rectangular format, the H5001 and H5006 in alternative geometries, and the H4652 as a rectangular variant closely related to the H4651.
The H5001 (high beam) and H5006 (low beam) are the dominant types for the 1967-1976 model years, with the H5006 being a 5.75-inch round unit and the H5001 a 4-inch square unit, both operating at 12V and conforming to ANSI standards for the US and Canadian markets.
Beginning with the 1977 Mercury Marquis, the high beam position shifted to the H4651 rectangular sealed beam, and the low beam shifted to the H4656, representing a change in headlight form factor from the earlier square and round units.
The 1978 model year is an exception within the otherwise consistent 1977-1986 period, reverting to the H5001 for high beam and the H5006 for low beam rather than the rectangular types used in surrounding years.
The H4652, fitted to the 1981 low beam position, is described as a variant of the H4651 in a 4x6-inch rectangular sealed beam format. Despite their close relationship, the H4651 and H4652 carry different designations and are not listed as interchangeable in the specifications.
All five sealed beam types listed carry ANSI standardization applicable to both the United States and Canada, and all operate on a 12V electrical system.
Because all units across both beam positions are sealed beams, physical form factor governs replacement compatibility; the rectangular H4651, H4652, and H4656 units are not interchangeable with the square H5001 or the round H5006 units.