Hot air from the outside vent indicates that the heater is "fired up", that's good. Open the panel to access the heater from the inside and check to see if the duct work is connected. If it's just laying in there, then there's your problem. Turn on the heater, if you feel hot air blowing inside the cabinet then it's got to be the duct work connection.
Good luck.
__________________
"PT Crew Members Since 9/2010"
2011 RAM 2500 HD 6.7L CTD Crew
2014 Prime Time Sanibel 3250