Coolant temps too low.
 

My coolant temps don't get high enough as I am driving at highway speeds. Once I stop and idle, they slowly creep up. The heater output seems to be affected by this too. Doesn't put out hot hot air, just warm air.

I just changed the water pump, thermostat, and radiator cap. Also flushed the cooling system and put new antifreeze in it.

Did I possibly get a bad thermostat from the parts store?

Also curious, where does the heat for the rear system come from? Is it the same heater core? The rear takes even longer to warm up and at highway speeds, doesn't seem to warm up but just a little.

Model year?

I did see a problem like this a 1970's Dodge Dart. The water pump's impeller was plastic and it lost its grip on the water pump's shaft. The temperature sensor remained cold and the heater didn't work. The coolant in the engine was probably very hot.

They probably don't make water pumps like that anymore, but I hope this helps.

I am going to say an 01 to 06 and broken thermostat

It's an 04 and like I said, I just replaced the thermostat. Maybe I got a bad one.