Plymouth GTX Dome Light Bulb Size

The Plymouth GTX dome light requires bulb size 257 or 1004 depending on the specific vehicle configuration. Data for the lighting components covers the range from 1967 to 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
1971	257
1967-1970	1004

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Festoon or BA9s base instrument bulb. 12V, 2W.	257
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. Variant of 1157. 12V, 21/5W.	1004

Other Locations

Additional Notes

The Plymouth GTX dome light used two distinct bulb types across its production run: the 1004 for 1967-1970 and the 257 for 1971.
The 1004 bulb is an ANSI-standard type recognized in both the US and Canada, operating at 12 volts with a dual-filament design rated at 21/5 watts.
The 1004 uses a BAY15d bayonet base, which is an offset-pin base that prevents incorrect insertion by ensuring only one rotational orientation fits the socket.
The 1004 is a variant of the 1157 bulb. Despite this relationship, the two are not always interchangeable, as physical base dimensions and filament ratings can differ between variants.
The 257 bulb is also an ANSI-standard type recognized in both the US and Canada, operating at 12 volts and rated at 2 watts.
The 257 is available in either a festoon or BA9s base configuration. A festoon bulb is held in place by contact pressure at each end rather than a base socket, while a BA9s is a small single-pin bayonet base.
The shift from the 1004 in 1970 to the 257 in 1971 reflects a change in socket type between those model years on the Plymouth GTX, meaning the two bulbs are not interchangeable across that transition.
The 1004 is a dual-filament bulb, though its use in a dome light application typically engages only one of those filaments, as dome lights do not require the variable-intensity function that dual filaments are designed to provide.