[steely]

Welcome and congratulations.
You're either missing bulb power or a good return (to ground).
You could try removing the lens and bulb, and inspect the inside of the bulb socket. If the bulb itself was dirty/corroded, chances are the socket could be too.
Use a pen eraser inside the socket base a couple of times to clean it.

You could also be missing continuity in the ground lead, or the power lead.
If you have a meter, do a continuity to gnd for the brown wire / socket side (should be near 0 ohms), or do a careful +12V check with the meter to the inside base pin (tape around the meter lead except for very end) with lights on.
If you don't have +12 there, you'll have to follow the wire into the car to find where you do get power.
There is a schematic on the main page (left side) for 78s - not sure if it is the same.
hope this helps.