No heat

My 1990 lumina had no heat when i bought so i just replaced the thermostat and flushed the coolant and that didn't work so today i changed the heater core and still no heat any ideas?

Assuming your heater motor is blowing air, maybe one of the vacuum "motors" isn't working. Move the hot/cold slider lever back and forth and see if makes a diiference. Also move the heater/defrost lever and listen if the air doors are moving.

i had this problem awhile back. changed the blower motor and that wasn't it. it ended up just being a [big] fuse under the hood (i'd have to check my owner's manuel to see which it was).

i replaced the heater core, thermostat, coolant, and just did the heater controls and still no heat

i'm still guessing a fuse. also, check to make sure you don't have a coolant leak (i know you just changed the fluid), but a leak could keep the heat from working if the level gets too low

Check the two hoses for the heater core and be sure both are hot when the engine is warmed up. If they are, then coolant is circulating through the heater core. If not, you may have air trapped in the cooling system from the t-stat/coolant flush and the heater core is the most common place the air settles causing a vapor-lock type situation and no coolant flow through the heater core. These cars have to be bled of air any time the cooling system sis opened using the brass bleeder valve(s) on the t-stat housing and/or coolant transfer pipe near the water pump.

Does the control head switch from floor to vent to defrost? The mode switching is controlled by engine vacuum (as is the cool to hot temperature actuator), so look for cracked/loose/missing vacuum hoses and/or a broken ball shaped vacuum storage canister on the firewall near the brake master cylinder.

I am not sure this is helpful but my lumina has heat but it does not work when I am sitting. After I turn a corner it seems to work. It stops when I am parked most of the time but usually resumes after reving up the engine.
I thought it was a broken door valve , letting the door swing from cold to hot air. But since it stops blowing when at idle and starts blowing hot when reved up ?
My chrysler had the same problem with the a/c . It was a broken vacuum valve on the door for the hot and cold air flow.