heat does not work

I have no heat in my 2000 grand prix. I have taken it to the mechanic twice now, they said the heater coil is fine and the first time they replaced the thermostat because they said it was broken and that should fix the problem. then a week later i took it back because the heat never came on. they then told me that i probably have a blockage in my engine coolant lines so i had my anti-freeze flushed out and then refilled. I still have no heat in my car and i am down $300. right now i am not concerned about the money that i have spent, i just want the heat in my car to work. i live in upstate new york and i am tired of wearing multiple layers in order to stay warm on my morning commute. does anybody have any advice on what i should do next. what else would be causing my heat to not work. all that is coming out is cold air so i dont think it could be the blower. i am a handy guy and i want to take a look at it myself tomorrow. any pointers on how i should approach this would be greatly appreciated... thank you..

Joe

warm the engine to operating temp, and then feel both heater hoses. The should both be relatively the same temp. If one is hot and the other is cold, your heater core is blocked.