Nice looking People !
Hope you see many fun miles in your future.
lights... before inserting a bulb, (2 pin, 2 conductor w/case ground), observe the condition and orientation of the contacts at the base of the socket. Are the contacts in good condition and not gauled from repeated installations? When you insert the new bulb, then twist it the few degrees to lock it in position, would you guess the contacts are aligning properly?
Sometimes, there is a phenolic type material holding the contacts, springs, and a 2 wire pigtail, and perhaps an additional ground wire to be fastened to the outer shell of the light receptacle.
IF a tab on the plastic of the contact holder has broken, connections will not align properly, causing the situation you describe.
Other times I have seen the pigtail twisted and bare, energizing both filaments at the same time.
Hope that might be of assistance...
redk