It will be helpful to figure out the wiring modification.
We can see two wires into the passenger compartment have been squeezed through the speedometer cable grommet.
What do they connect to?
Power to the electric fan is a heavy red wire. Likely this is the red wire from the relay above the rain gutter.
Therefore the heavy yellow wire probably is the power feed to the relay. Where does the other end of the heavy yellow wire connect?
Here's a good photo of an unrestored '67 Valiant as a comparison.