The approach of reutilizing existing parking lots is great, but some spots without solar panels should be reserved for the solar self-charging cars that are coming to market in the next few years:
The Lightyear and Aptera can charge up to 40 miles of range per day on their built-in solar panels, and the Sion can add about 15.
With that much charging capability, the built in solar panels can provide energy for a vast majority of the daily miles driven in the vehicles, and not just function as a backup for when plug charging is unavailable.