Froggy55:
I was told it was the Montcenis pass road, which was the main way to Italy from France before the Mont Blanc and Fréjus Tunnels were bored. The Relaxe cab ha a proper standard heating system, but certainly not as efficient as todays heatings. The “M2” bonneted cab had an optionnal efficient heating/defrosting. I remember that, when I started driving in 1979, some drivers filled a big tin can with sand and alcohol, and kept it burning during the night.
It looked like Mont Cenis to me but someone in the comments questioned that.
I have done both but only with the car over Genevre.
As regards the heating, they seem to be double manned so maybe they cuddled up together. 
A burning can all night, seems a bit extreme and what about the fumes?
Reminds me of when I had a bright idea to stop my Albion freezing its windows parked outside home.
I put a paraffin heater (the flat round type usually pushed under the sump to keep the oil warm)
on the bonnet but in the morning the windows were black with soot. 
