Had to take the heater core hose a loose to get cold air the mechanic say i need a some kind of switch to that changes from hot to cold

Changing the thermostat was a good start.
Your issue might be a clogged heater core:
You need to warm up the engine again and turn on your heater. If it feels cold, then lift your hood and look for your heater hoses. There should be two small hoses going from the engine to the firewall on the passenger side. BEING CAREFUL, feel the hoses for being hot. You just need to touch the hoses and see if it feel warm to hot. If only one hose feel warm or hot and the other hose does not, you have a clogged heater core in the dash. If both hoses fell hot, you have a failure under the dash. The most common failure of the heater is the air mixer door on the heater/air conditioning assembly under and inside the dash. The mixer door has a tendency to tear up the fiberglass assembly that holds the door.
If it is not the core or the mix door ... then some people have had issues with the controls (at least on the same year Grand Am) ... see link
2003 grand am heater gets cold then hot then cold
Helpful?
Good luck on your troubleshooting!

Start your vehicle and let it reach normal operating temperature. Then, look for heater hoses that run into firewall. Carefully, grab both hose, they should be warm/hot. If only one is hot,could be clogged heater core, T-stat could be bad or mechanical component that allows warm coolant to run thru, is not working properly.
When you change setting inside, your heater and A/C panel, from cold to hot setting, do you hear anything ? There's a flap that opens and closes the coolant passage. This can be a mechanical or electronic operated component.

ok i have found a number of newer cars and trucks seam to have this problem and if the engine is running at the normal temp and if you move the temp leaver from hot to cold and you can hear the leaver move then your looking at a plugged heater core, if you have any kind of mechanical ability this is not to bad to fix. You will have to run the engine and at the fire wall you will see 2, 1" dia rubber hoses that go to the heater core get the engine up to temp and you will need to touch both hoses one at a time to see if one is hot and other is cold mark the cold one because that is the out side and thats the one you will need to push water or a low amount of air into. you will need to remove to hoses and push water or air into them then once this is done get some CLR and pour into the side that was hot and let sit for a bit ( reason is some of the new antifreeze out there crystallizes and plugs the heater core) now after sitting for a bit wash out with water eg garden hose or a small water bottle and push into the hose pushing out the junk then hook back up hoses and run engine and top up antifreeze. i have done this to over 20 vehicles in the past 3 months.

What kind of setting do you have? is it turn knob, push button or something else?

Explain how you had a mechanic work on the problem
& not resolve

If the hose from the water pump is hot & the return from
the heater core is not ,then there is a control valve somewhere

Some mechanic that is

We don't have a data base to look up that ,we are at home like
you,social web site

If you are under Warranty, then it is the Dealers job to fix it. What you need to check are the water lines coming out of the cabin area called a Firewall. Both heater hoses should be hot after the engine is hot and the heater is on.

If one of the heater hoses is lukewarm or cold, then the problem is the water is not going through the heater core. The core could be plugged or the heater controls which allow the water to flow are not turning on.

If both hoses are hot, then you have a ductwork problem inside the cabin. The heater core is hot because both hoses are hot, but the air is not flowing around the heater core box. This is why the air remains cold.

A mechanic should inspect the dash controls and ductwork flaps. It sounds like you need to go to a different Dealer on this. The first Dealer should have verified the repair before giving the car back to you. If you use the same Dealer, then the mechanics ID# should be on the repair records.

You should ask for a different mechanic.

flush heater core if one hose is hot and one cold there is blockage in heater core back flushing heater core should fix problem

Yes clogged heater core may be the answer in your case .

Check both heater hoses under hood to see if they get hot, if they do the heater is fine. need to check to see if the selection doors inside the unit are opening and closing.

I experienced the same head ache. First attempt was to replace the thrmostat and after taking it apart I realized that there was nothing wrong with it. I then removed the plastic cover/panel located under the dash board on the passenger's side. (only a few 4-5 smalls screws) Removal will enable you to see the on/off switch/gate moving/rotating as you turn the knob to hot or cold positions, on the control panel. If that switch/gate seems to work ok, then the trouble is within the heater core. Many mechanincs suggested that I replace the heater core. A very expensive proposition.
I checked the heater core supply/intake hose and found it hot to the touch, while the return hose was nearly cold. It was definitely a heater core blockage. I removed both ends (supply and return) hose connections/clamps. They are located near the fire wall in the engine compartment. A special tool is very helpful to remove those clamps easily and to replace them as well. It can be done the old fashion way but this tool was a life saver and reduced work time to just minutes. (Tool is $70.00 bucks at auto parts store,so I borrowed mine from a mechanic/friend)
I then hooked up a garden hose to the return end of the heater core and reversed the flow of coolant/water through the heater core. My truck is a Sierra 2000 and I refuse to see how a heater core could get clogged up with clear coolant alone. To my surprise a flat piece of rubber about the size of a quarter blew out of it and the flow of water became unobstructed. I reconected the ends of the hoses to the heater core, replaced the lost antifreeze & everything returned back to normal. No money spent for something most mechanics suggested I replace control valves/switches ($280.00) replace heater core ($500.00) etc etc. I am glad I was curageous enough to tackle this myself and save a few pennies. Good luck, and I hope this will work for you.