No heat to passenger side

I just got my Sebring Austin Healey and have a question I'm hoping someone can help me with. We are getting no heat to the passenger side. Plenty of heat to the driver's side. We opened the flap to the passenger but nothing comes out. Any ideas?

Maybe...
If it's the common Mustang II without air conditioning they are pretty simple. When we travel, I plug my side with some batting while the wife basks in the heat. So get under there and look for blockage. Then test the control cables to make sure they are not frozen. Does the defrost work ?
Just depends of what system you have to diagnose any further, but seventies stuff is all pretty basic.

I happen to be working on my heater control box right now since I had to pull it out to replace the heater core. My car was missing a piece of duct (part of the Mustang II assy) that connects to the outlet on the driver side and diverts some of the flow to the passenger side.

By chance are you also missing this piece?