i looked up FSM info for a 2012 model here,
Factory Service Manual
The Factory Service Manual indicates that there are trouble codes (DTCs) for the A/C and Heater systems, although there will not be a warning light on the dash. So reading the DTCs thru the OBDII connector would be a good start to determine the issue, e.g. Dealer MUT-3 tool, OBDII scantool, etc.
There are DTC code charts for both the controller (dash knob controller assembly) and the ECU (located under the rear seat).
Also there is a troubleshooting guide for poor performance of the Heater,
Possible Causes:
Damaged harness wires and connectors
User’s incorrect operation
Limitation due to main battery remaining capacity
Insufficient volume of water heater fluid
Malfunction of the A/C-ECU [under rear seat]
Malfunction of A/C & Heater control unit [dash knob assembly]
There is a service bulletin with some corrections that has the power circuit diagrams,
http://mmc-manuals.ru/msb/BASE/003/2012/MSB-12E55-501A/E/MSB-12E55-501A.pdf
My first guess is that there may be 2 different circuits for powering up the controller assembly depending upon the mode, e.g. READY or Charging/Remote, so verify the fuses and relays for the heater section to rule out a power issue. Then ring out the low voltage control harnesses for rodent damage, corrosion, broken wires or connectors, etc.
power circuit diagram showing connectors and wire colors,