Thanks guys for the comments.
crazy 989, you're right about the windshield/rearshield/side windows. I forgot to give some depth, so the computation of refraction was completely biased. Below is a render with a proper thickness (3mm) and correct IOR (~1.52)
Nik, thanks. I'm not ashamed to say that I can see my skills getting better, because I started from very very low :
http://www.smcars.net/forums/work-pr...ighlight=xsara. First CG car ever
LOST, I'll keep it going, for sure. Contests are really helpful for me to free time to properly progress on a given model.
The render below is 4 times bigger than previous ones, to help see details and bumps and problems.