You're going to have to get out your multimeter and check for voltage in the wires of the light that isn't working...or possibly the connection from the switch to that light may be suspect....
AS used a lot of those twist-type connectors back then...and they are JUNK, especially after all these years...the wire ends eventually get some 'varnish' like coating after time, and they don't connect very well...
Check your connections, and then use some quality crimp type connectors after cleaning the copper wire to get it 'bright' again...
That's my story, and I'm sticking to it...