I have the Radio out of my 56 to change the Wiper Motor to Electric. I have a Vacuum Line coming from each side of the Dash which are for the Heater with the  Vaccum Valve on the Engine. There is a "T" in the middle where 3 Lines connect together. Where does the Line comming off the bottom of the "T" connect to? The Line from the Passenger Side is connected to the Vacuum Valve on Engine I assume. Where does the Line on the Driver Side connect to? Can't tell as it disappears on top of the Heater Box. I wonder if that "T" Line is to go to Windshield Wiper Motor? It was disconnected under the Dash when I got the Motor out.

  

Mine disappears into the heater box too. I found that the vacuum to the device is activated by vacuum only when your move the lever to add heat. If you check the hose at the valve on the engine with he engine running there is no vacuum. Move the lever then there's vacuum to operate the valve. It happen inside the heater box with the lever cable.
My tee went to the vacuum wiper motor so if I were you I'd put a cap on the tee or replace it with an inline splice from a "Help" assortment from an auto parts store.
Tell us how you like the electrics after installation. Are they Newport and did you buy the extension to use your standard wiper knob. Thanks

56 Vic, B'Ville 200 MPH Club Member, So Cal.

Thanks. Kinda thought it might be for Wiper Motor. I bought a Newport Wiper Kit along withe Extension about 10 Years ago for my Sedan. 2 Speeds.Works great. With the Extension You can use your Stock Knob and no One can see its been converted. Interestingly the Price has not changed much since I bought the last One.