I've only got the 69 wiring diagram, so take this with a grain of salt. I think the 67's would be the same but you never know.
The original orange wire in the engine compartment that runs to the blower motor is taped up and left in the wiring gutter on an A/C car. The opposite end goes thru the firewall in the right bulkhead connector, and would have ended at the 3 speed fan control switch connector behind the center of the dash. Note that the fan control switch is only a three speed control switch, lo, medium and high. This connector originally had 5 wires on it, 2 orange, 1 light blue, 1 yellow and 1 brown. The brown wire is the main power feed to the blower motor and runs back to the Fuse panel. The yellow, light blue and other orange wire run to the blower motor resistor. The yellow wire is the hot side for one speed, and the light blue is the hot side for the other speed, and the orange is the return from the resistor going to the fan. This connector is usually cut free and discarded when the A/C wiring harness is installed. The A/C wiring harness uses the brown wire as the main feed to the A/C extension. The brown wire is connected to the lo blow contol switch which is normally closed and supplies power to the rest of the system except when the Temperature Control lever (TOP one in a 67/68) is in the OFF position. In the OFF position the switch is open and the fan will not run and you can't energize the clutch on the A/C comressor.
From the LO BLO Switch a tan wire runs to the 4 speed A/C fan control switch. From here 3 wires, 1 yellow, 1 light blue and 1 light green run to the blower motor resistor, and an orange wire runs to the blower motor relay. The orange wire picks up the blower motor relay in high speed, which in turn closes a contact on the relay, and power comes through the larger #12 AWG wire from the horn relay to feed the fan directly off the battery.
There are a few other connections and switches that are involved in this system that will probably prevent the fan from running with the AC slides in certain positions.
This is the under hood wiring of a non A/C car: (note 63K file size)
This is the under dash wiring of a non A/C car: (note: 337K file size)
I've got the 69 A/C wiring diagram if you need that. Don't have a 67/68 version.