I'm not sure sun is the ripening component. My guess would be the increase in sugar content and the maturation of the fruit.
Discussion
Temperature is major component. Optimum temperature for ripening on the vine is 21-24C (70-75F). Above or below this and the process slows down. We're averaging 18C (64F) here, in what is meant to be summer! Hence my call for the sun to come out.