As mentioned previously there is a tiny switch in place that is activated by the lift mechanism when you crank the vent cover open.
We have a '74 Sovereign
and the previous owner must have ran into the same problem that you are having and found out that the little switch was faulty.
His solution was to by pass the switch and drill the light cover and install a small toggle switch to operate the fan.
I think the installation of a toggle switch may be a positive thing as you can open the vent and then choose whether the fan would be on or off.
At some point you may want to consider purchasing new light covers - we did and the difference is unreal. We also converted the original incadescent bulbs to LED'S and the difference is like night and day.
There is a shroud that circles the fan. Our shroud was broken up and we were fortunate in being able to buy a used shroud that was in good condition.
These are some of trials and tribulations of owning an older Airstream.
Good luck with your repairs.