Outside Cold Air coming through at all times 96 Dodge RAM B2500 Van

[Meridivs]

Since winter came around we noticed a problem with the HVAC system on this van:
- when the AC/Heater is off cold air is still coming in through the vents - the higher the speed, the more cold air we have rushing through the vents
- if the Heater is on we have hot air while idling/travelling at low speeds up to 50 mph but the air gets cold when we get up to highway speeds

Engine temperature stays constant the whole time, just inside air temperature changes with speed.

Is there a door/vacuum line that regulates the air coming from outside?

How can we find this HVAC door/line before we freeze completely in this van?