If you have a single lever faucet in your sink, you know how car heater - A/C works. Hot water & cold water enter the faucet at a constant temperature. Rotating the lever changes the amount of hot/cold water being mixed together to create the temperature of the water flowing into your sink. If the hot inlet gets clogged/stopped up, only cold water will come out of the faucet regardless of how you turn the lever.
The air box inside your dashboard has both heater [hot] and A/C [cold] areas. Doors/flaps controlled by servos separate each area. When you set the temperature on the dash, you regulate how much each hot/cold doors open/close. It would appear that the servo which controls your driver side heater door doesn't work anymore. You are only getting cold air without any hot air being mixed in with it, as is happening on your passenger side. To find out what is happening, you have to open the dash.
Wish I had a better solution.
The air box inside your dashboard has both heater [hot] and A/C [cold] areas. Doors/flaps controlled by servos separate each area. When you set the temperature on the dash, you regulate how much each hot/cold doors open/close. It would appear that the servo which controls your driver side heater door doesn't work anymore. You are only getting cold air without any hot air being mixed in with it, as is happening on your passenger side. To find out what is happening, you have to open the dash.
Wish I had a better solution.