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.