I had little heat in my '07 Grand Prix. I flushed the system of dexcool and changed the thermostat. Heater hoses were hot to the touch. Still had little heat. New coolant was clear of gunk. Starting to think I had a bad core.
Read up on how to replace a heater core and thought, there has to be another answer.
Since one heater hose already has the flush "t", I cut the other and put in another t. I hooked up the garden hose to to one heater hose and squeezed the nozzle - out came rusty colored pudding. It ran clear in a few seconds. Then I hooked up the nozzle to the other hose to back flush. Surprisingly, more gunk came out. Hooked the hoses back to normal, refilled the coolant I lost, bled the system, and you could cook a roast in the car now.