Its a little cold in the D3

[HPsauce]

That unit is very similar between D2 and D3 and incorporates the auxiliary water pump, which has a habit of failing and getting very noisy.
On both my D2's I've had to disconnect power to it as the noise got excessive, BUT the heating still works though takes a little longer to warm up in the cabin.

However, the main point of this is that the pump also drives the circuit for the "Rest" heating function, which I have never ever found a use for.

BUT it's worth testing - get the engine warm, turn off ignition and then try the Rest heater. The HVAC should start up; see if it's warm or cold in that scenario.

As noted above, it's a bit of a mission to change the unit (and expensive) so you do need to be sure that's absolutely necessary.