I assume you are asking about a bulb for the conventional lamp, typically called a teardrop lamp. If so, the 6-volt bulb you have is a number 1133. The 12-volt equivalent is 1143. If you can't find it, an 1156 is a common backup lamp bulb that can be used. 1156 has a smaller glass bulb but fits the socket. There are a variety of substitutes that can be found (e.g. LED types).