No heat with 2007 Grand Cherokee

Here's the symptoms
:-The heat does not work.
-The fan is blowing, the fan speed is adjustable and blows.forcefully, just Cold.
-Slow to warm.up
- Heat blows cold air no matter where you set the temperature Guage.
- Car never overheats.
-Engine temperature, appears.normal, never to high never to cold.
-Replaced.drivers side blend door actuator.
-Replaced thermostat
-Topped off antifreeze
-Tried briefly reversing the tube's going to heater core, in an attempt to blowing any potential.blockage out.
- I did see gunk on bottom of radiator cap, gel like gunk.
-No antifreeze smel in vehicle
- I replaced 1 blend door actuator I'm not sure if there's 2 or not.
-When car is cold and you start it the radiator hoses BOTH upper and lower suck closed and flatten drastically , like a vacuum is in the system.coolant.system, when the car warms up, that goes away. And the radiator hoses appear normal.
-This vehicle does.not have dual zone climate control. One setting fir whole car, so I don't believe there is a second blend door actuator.
******** WHAT DO II REPLACE NEXT????? ***********
-I'm fixing this myself, so don't say bring to mechanic.
-PLEASE TELL ME , WHAT TO DO, WHAT TO CHECK,??
-THIS IS A JEEP GRAND CHEROKEE LAREDO (3.7L V6 4 WHEEL DRIVE)
Jne2024@gmail.com