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 are

     width 1.0 inches
     length 2.0 inches

See donsbulbs.com for detailed specifications of the bulb you are replacing.