Where is the external air temperature sensor on 1999 C280?

What color wires lead to it? It's not shown on the wiring diagrams I have for the A/C.

The climate control diagnostics report the external air temperature sensor as OPEN. The temperature indicator below the speedometer reports -36F.

The heater doesn't work unless the temperature is set to HI, one step above 92. I think the reason is that the external air temperature sensor is defective or disconnected.