2002 Kia Sedona Cooling issue

If this is a similar design as used on the Chrysler minivans, they have plastic quick-connect fittings with o-rings that shrink and leak. Replacement, "improved" lines have two o-rings that shrink and leak. The permanent fix, which is much less expensive is to remove those lines and run bulk rubber heater hose from front to back and attach them with standard hose clamps. If you leave the original hoses bolted to the body, you can use nylon tie straps to hold the new hoses to them.

The rear heater might be hard to purge the air out of. Use a cooling system pressure tester to build pressure in the system, or just wait for the engine to warm up and build pressure on its own, clamp closed one of the new hoses, then loosen the clamp for that hose at the rear. The pressure will force coolant to go into the heater core and push air out by the loosened clamp.

You're only looking at a dozen feet of rubber bulk hose. Same cost as a few gallons of gas and they'll never rust. I would expect the repair to take less than an hour if you're crawling under it on the ground.

I can't say specifically about your vehicle, but there are some designs that will cause engine overheating if the heater core is bypassed. The restriction around the thermostat is needed for proper operation. Never looked into whether that might apply to a rear heater.