Originally Posted by family wagon
Maybe remove all the bulbs, then re-install and re-test one bulb at a time? At first I was thinking install a single bulb, then another (now two are installed), then another (now 3 installed) and so on. But it might also be helpful to test each bulb position in isolation, ie with no other bulbs installed at the same time. Those bulbs that don't light alone, but do light in combination with other bulb(s), must have some wiring problem between them and their battery or ground connections.
That's a great idea. It essentially lays out the checks for all the bulbs and the sockets for you.
I'm not sure how your bus is wired, but with mine, the top and bottom lights are all wired the same. They all are running and stop lights and require double filament bulbs.
Look inside your sockets and see how many contacts you have at the bottom. Double filaments should look like this with two contacts.
While your single filaments should only have one contact in the center. Like this.
Let me know what you got.