Q:

On a French car, what does "eclair" mean, please? 

I soon hope to buy a Simca, with "eclair" as one of the dashboard controls.  Is this a reference to the lights? 

A:

Yes, it is indeed a short form of éclairage used on some Simca dashboards. 
Here an Ariane 4 SL 1959 model dashboard:

and here is an excerpt from an add of the Vedette V8 1956 model:

The buttons labels are:
- CHAUF : Chauffage (heater) on the Vedette V8
- CLIMAT : Climatisation (air conditioning) on the Ariane 4
- START : Starter (choke)
- ECLAIR: Éclairage (lights)
- AVERT : Avertisseur (horn)
- ESS GL: Essuie glace (wiper)
- DEMAR : Démarreur (starter)  
Photo credits: https://simca67etanciennes.jimdo.com/simc-adh%C3%A9rents/patrick/ and http://www.auto-pub.net/Simca/Simca_Vedette_56/Simca_Vedette_6.jpg

