What is the proceedure for back flushing the heater core on a 94 z-28. I had the radiator flushed, and since then there has been no hot air from the heater. I have put in a new thermostat and radiator cap, and I have tried squeezing the hoses to remove air, but still only get cool air. I was told there may be a blockage at the core and that I should try to back flush it. Any help on this, or any other suggestions are appreciated. Thanks.