1956 courtesy or dome light question

[mike peters]

The dome lights on my 56 Seville are a double contact, single filament bulb, and I can't figure out how these  are supposed to work. Each contact on the base of the bulb has  a wire lead.   There is NO ground wire coming from the brass base of the bulb to complete the circuit.   My shop manual is silent on this circuit.  Does anyone know how these dome light circuits work in principle?   I have rewired the dome lights with conventional single contact bulbs to ground out through the door post switch and now they work great, but am curious as to how the originals operate.   Thanks
mike peters

[Martin Michaels]

Mike ,One wire is hot all the time,the other wire is the ground.The circuit is completed when a door is opened.The switches are normally open,does this help? Marty

[mike peters]

Marty:  I tested a bulb on a battery and it worked!   You're right!  The obvious did not occur to me. I've spent the last 30 years working on 6 volt cars ( Model T through 52 Chevy) that use single filament bulbs which ground through the  light fixtures.  Gotta get with this modern stuff.
It's a T 90 bulb.  Will look for replacements at the local auto parts store. 
Thanks Marty!!