How do you flush the front heater core in a 2005 kia sedona ex?

If the fans are blowing air out the right places then that part of the system is ok. Lets focus on the no heat. With the system warmed up, check both the heater hoses to see if they are hot. If they are not, then there may be a restriction in the hoses or the heater core its self and the system will need to be flushed. Also watch this video https://www.youtube.com/watch?v=9orWU9B5ZQ0

You have heater valve problem ,see if the valve open and close ,all the best.

Also, besides coolant level, the heater core may be restricted and the temp blend door may be inoperative .

There is a separate button on the lower dash labeled RR HTR for the rear heater. If you do not engage the switch it will blow cold air.

it is your blend door motor

Check the metal part of the heater core lines at the firewall if you can get to them and see if one side is noticeable colder than the other. At least one of them should be hot or you have a bad thermostat. If both are hot than your hot/cold actuator inside is having a problem

This could be a serious problem. Check your coolant level, then check the rear heater lines that supply coolant to the rear heater. They run under the van, over time they rust and leak. Because it is under and to the rear of the van it is hard to notice, i.e. coolant puddles and smell. But please note that this engine has aluminum cylinder heads and does not take kindly to being overheated.

check the air ducts make sure there isnt a break

Location of components...

many vans have rear heater cores for added passenger comfort, so this is not unusual, it is very possiable either the hoses or there connections to the rear eater core are leaking or the heater core itself is leaking.