No air blowing out of my heating system and ac unit in my car

same problem, hot air on driver side, cold air on passenger side. casting sand is in the motor. i had my heater core,motor and radiator flushed out, in fact heater core flushed twice,2 new thermostats installed new antifreeze. mechanic told sand came out of heater core and radiator. this motor has 2 thermostats in it also had sand on them. saturday morning 7 degrees outside rode around, had to put the heater on low, it was that hot in the car. total bill for 2 thermostats ,heater core flush ,radiator flush, engine flush, new antifreeze came to $330.00 car has great heat, problem solved.