Sounds like there's an intermittent open circuit either at the outside temperature sender's two-position connector (the sender itself is fitted to the header panel's passenger side vertical support and is a black plastic cylinder with a pointy cone-shaped end on it) or the wiring running between it and the multi-pin harness connector at the blower motor. Said connector is located on a bracket that's right next to the blower motor. Between the digi-dash and said multi-pin harness connector, there is one continuous run of wiring which is mostly safe from harm inside the vehicle save the two or so feet sticking out past the firewall by said blower motor. That said, the wiring between the temperature sender and the blower motor runs along the passenger side main body wiring harness.
A completely open circuit will cause the digi-dash's message center to display -40º F (-40º C) continuously.
If you have ATC automatic temperature control, you'll have two identical outside temperature senders fitted to a common bracket. Both senders are electrically interchangeable and you can swap the sender connectors for further diagnosis.
As for wire identification in the engine bay, the outside temperature sender has a dark blue/yellow stripe sensing wire and a grey/red stripe ground wire. The ATC temperature sender has a light blue/orange stripe sensing wire & pink/black stripe ground wire.
Hope this helps.