I assume you're referring to the cowl vent. You can start by pulling the grill/cover from the cowl and look to see if it's buried in debris and clear it out. There are two small nuts that hold the vent in place, you can remove these and lift the vent off to access the rods that actuate the vent. They may be rusted in place, so try your favorite penetrating oil to free them up. If you can't free them from the top side it's time pull the heater core. This will give you access to the operating mechanism, from here you can get a better idea of what's frozen or binding.