A/c is working for my 2000 gmc sierra. but the heater is not working any suggestions

Make sure you have good ground not only to the engine but from either battery to body and chassis or engine to body and chassis.

did you check the fuses and did the motor come with a new heater resistor or is it separate

check fuses and relays under dash and under hood the ownser maunal will tell you were to find them

sounds like you have a bad actuator motor that is what moves the door in heater box to change from hot to cold

sounds like a switch as common error oboth sides unlikely

Check in the one heater hose will be a control valve. Check the operation of the valve(this is the heater control valve). If the lever on it appears to be moving. Take the valve out and manually check that the valve is opening and closing the whole way. If not replace the valve and recheck. If it is the heater core may be clogged not allowing the hot coolant to flow thru so you can get heat.

After replacing the thermostat I pulled the heater hoses off and blew air through the heater core at firewall. Can only blow in left side as directional valves will prevent opposite. Chunks of stuff came out with coolant. I then put a piece of hose on the left side and shot water through until it came out clear.
Put it all back together and heat works fine.

make sure antifreeze is full. run engine 10 min. verify engine is around 195 degrees. if not, try replacing thermostat. if it is running at 195, then maybe heater core problem or heater control not functioning properly

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.

ur gonna have to tear the dash apart to get to the heater core, once u access that u can take the pastic housing off and reconnect the hoses