'86 Jeep Grand Wagoneer heater door

Hi,

I have a 1986 Jeep Grand Wagoneer (full size, not the “Cherokee” Wagoneer), very base model, no a/c. There’s a door under the dash on the side of the heater unit that (I think) controls fresh air to the passenger compartment. When that door doesn’t close, there’s no heat in the winter, or if there’s heat it’s overpowered by cold fresh air. I suspect the door is controlled by vacuum, but I’m not sure. I currently have a piece of rope tied to the lever that operates the door, but it’s not exactly easy to adjust. I’d like to make a proper repair, but can’t find any resources on YouTube or Google that address this particular issue. Any help?

Thanks,
Jason