There should be a hole on the lower passenger side firewall below the blower motor and just a little to the right (looking from the front of the car).
Can send photo if this doesn't locate it well enough for you. Should be two holes in the grommet. One to pick up vacuum from the engine to supply vacuum to the HVAC unit. The other vac line comes back from the HVAC unit to the vacuum actuated heater valve in the engine compartment. At least that's how I remember it on my 71 Charger w/AC. Yours may have a cable actuated heater control valve. I can't remember when they changed to that (I think in 73).