rear heater in MX is unacceptably bad. Mine has been in the shop a few times and they tell me "try this setting; turn of that." doesn't matter. it's still no good and apparently no way to fix.
specifically, at 35F today I preheated the car for 45 minutes. then used rear heat on MANUAL/HIGH. I kept checking and got cold air out of rear pillar vent UNTIL after driving for 55 min. Tech said to turn OFF "range mode" so I did that and tried on the way home. same deal. except worse. the car was in my heated (70F) factory and plugged in. when I drove home the rear vents were still blowing cold air when I got home. No way the battery could be very cold when it was in a 70F space for 8 hrs!!
What fan speed were you using on the rear heat? I've found that anything over about 4 overwhelms the tiny heating element and the air coming out of the vents feels cool instead of warm. Someone else reported that their rear heat doesn't function at all, so you may have a problem with the system. Tesla should be able to see how much power the rear heating element is using and determine if it's functioning or not.