My foot heater vent works fine, blows hot air into the footwell on both sides. However, on the passenger side, there is also a huge air leak of some sort that allows frigid outside air to blow into the footwell on the right side from under the dash. Any hints on where the leak might be or how to plug it? It's extremely cold driving down the highway in winter, just in that one spot.

 

There is a wiring grommet on that side where the main wire harnesss goes through the firewall. I would check to see that it is not displaced and allowing cold air in. I am assuming of course that the footwell is not rusted out with a giant hole in it.

 

The air intake for your HVAC system is also in that area, perhaps a bad seal between the intake duct and the body of the car?

Other than that, are you sure your door seals are sealing well? You could have a funky backdraft blowing air from the door seal forward into the footwell and circulating around in there