The 1157 is the sole bulb type specified for the White/GMC WIS across all listed positions and model years, covering both the front and rear turn signals from 1988-1991.
The 1157 is a dual-filament bulb, meaning it contains two separate filaments within a single envelope, allowing it to serve two distinct lighting functions at different wattages.
The 1157 operates on a 12-volt electrical system and draws 21 watts on its high-intensity filament and 5 watts on its low-intensity filament, reflecting its dual-function design.
The 1157 uses a BAY15d bayonet base, which is an offset twin-pin mount. The offset pin placement is a deliberate anti-misalignment feature that prevents the bulb from being seated in an incorrect rotational position.
The 1157 carries ANSI standardization applicable in both the United States and Canada, indicating the bulb conforms to a recognized North American specification rather than a proprietary or regional-only standard.
Because the front and rear turn signals on the White/GMC WIS share the same 1157 specification, the bulbs for both positions are physically interchangeable with one another.
The BAY15d base of the 1157 is not compatible with single-pin bayonet sockets or other base types, so substitution with single-filament bulbs sharing a different base configuration would not be a direct fitment.