The BYA15d type
incandescent bulbs are used in
tail-lights and turn signals where two levels of light is required. NOTE: the BAY15d bulbs have
asymetric ears to insure the correct positioning of the two power feeds on the bottom of the base. In that series the case is ground and the two contacts
provide two sources of power.
Typical bulb numbers for this group
are the 1016, 1034, 1154, 1157, and 1176 whose physical dimensions
width 1.0 inches length 2.0
See donsbulbs.com for
detailed specifications of the bulb you are replacing.