The heat is very weak on drivers side, I'd say about 85 degrees, and passenger side is completely cold. Any ideas what could cause this? I'm planning on a heater core flush to improve the temperature but I don't understand how no hot air is getting to the passenger side since this car doesn't even have dual climate control. Any help is greatly appreciated.

 

After doing coolant flushes in the past, I've had issues with the heater not working and burping the system has fixed it. Usually remedies itself pretty quickly after all bubbles are out.