When I acquired my 63 Bird, the vacuum line from the manifold to the heater control head inside the car was broken/missing. My first instinct was to wade into the Electrical Assembly Manual and start taking things apart so I could access the routing for this vacuum line, which enters the firewall and then routes over the top of the blower assembly. But that quickly sized up to be a ridiculous amount of work for the job at hand. I decided to try and fish some mechanics wire through. I pushed about 4 feet in, figuring it had to pop out somewhere. Knowing the vacuum line is supposed to route over the top of the blower housing, I tried to push the wire up and toward the driver's side:
Again, kinda obvious stuff, but hoping it saves someone a little bit of work along the way!