When I turn on the heater, the front vents blow hot air, but the rear vents blow cold?

Hello, yes there is an entire rear HVAC unit. Unfortunately, service information is starting to service the A/C Evaporator, Mode Door, or Blend Door the rear HVAC unit needs to be removed from the vehicle. It looks like there are A/C lines that run under the vehicle and the rear HVAC unit is in the passenger side rear quarter section above the right rear tire well inside the vehicle. I will post diagrams showing the system. So, all of the refrigerants must be evacuated, the cooling system must be drained, battery disconnected, and the vehicle raised on a lift to unbolt the A/C lines before even getting the rear unit out. But I will post all the diagrams showing the whole process.
But this might actually be a wiring issue too, it saying here that the Front Blend door actuator is connected to the rear blend door actuator, so they move together when switching from hot to cold. I'll see if I can find where these wires run and maybe it will end up being a broken wire. Although the Blend doors do need to be calibrated, there might be a diagnostic trouble code stored in the HVAC control module if an issue is detected. In this case there should be a code stored. But you will need a scan tool that can scan the entire vehicle systems and not just the engine computer. I will post further information.
What you can do, is in the 4th diagram you can see where the rear HVAC unit it located, go back there and listen for any blend door movement while you have someone else up front change the controls from hot to cold and then back again, You should be able to hear the rear blend door moving, or if the actuator has stripped its gear off or broken inside you should hear some noise from that as well. You should be able to do this with just the Key On engine off, so you can hear better. If you don't hear anything at all, it's possible there is a broken wire somewhere. Or sometimes these doors get stuck but try that first and let us know what you hear. I will try to see where the wiring runs for the rear unit. It's most likely under the carpet somewhere.

Okay, service information seems to be incorrect in some places here. It does look like you can access the Rear Door Actuators without having to remove the entire HVAC unit. I'm not sure why they have it listed differently like that, but it's good news.
But you will have to take it somewhere to have the rear blend door calibrated. The actuators are on the backside of the rear HVAC unit, so when you remove the cover, they will be between the HVAC case and the right rear wall of the van.
I have added the access to the rear HVAC as well the last 5 diagrams.
Sorry for the confusion, the service information is spread out in different locations. Diagrams 7 to 15 are what you will need to get to the Rear Blend Door actuator. The door actuators are actually interchangeable according to service info, The Blend Door and Mode Air door actuators are the same part. So, you could try switching them around to verify the wiring.

https://www.2carpros.com/articles/replace-blend-door-motor