Ambient air temp sensor reading -40?

I had the same issue with my 2014. The cause is an open circuit in the two wires to the sensor. The polarity doesn't matter because there's just one component in the sensor. It's a simple temperature-dependent resistor. Those are extremely reliable, except, of course the one on my truck, but mine was a crash rebuilder that was hit in that side. -40 degrees is the default value for this, and a number of other temperature sensors, such as engine coolant and intake air, when he circuit is open. I found a used sensor in a salvage yard.

The diagrams for this sensor are very secretive and show just one wire going to it, which we know can't be right. If your truck uses door modules in the two front doors, the sensor goes to that. If you don't have door modules, the sensor is hard-wired to the Body Computer under the dash.

Since I can't find a connector view for your sensor, the drawing below is for my truck, which is likely the same. The sensor needs to connect to two violet wires, one with a light green stripe and one with a brown stripe.

The sensor isn't part of the mirror wiring, it is in a small trim panel under the left mirror and is an individual part. 68457701AA. It uses a Black wire and a White wire. It has it's own 3 pin connector and harness that goes through the mirror housing to the interior but only has 2 wires in the connector. No voltage on the wires as it's a simple thermistor that just plugs into the system to read the temperature. However Dodge decided to make things interesting and also used the black and white wires to feed CANbus signals to the BCM from the door module. If you remove the inner door panel you should be able to locate the wiring for the sensor where it comes from the door module. Did the replacement mirrors include a sensor? Do they have a wiring diagram? Attached are the diagrams for the two versions of the wiring (base or premium) for the mirrors and the colors of the harness wiring for the sensor in both as well.