Ask Keith what he thinks http://www.myswag.org/index.php?topic=27176.950 & previous page.
It sometimes seems that my role in life is to serve as a bad example.
My camper's nine expensive thin film solar panels all failed after 3 years because the "protective" coating on them gave way. The enemy of all solar panels, thin film especially, is heat. I would not expect long life from a thin film panel installed with no air gap underneath, especially on the bonnet of the vehicle. Unless you go with one of the best and most expensive brands, like Solbian, the affordable flexible panels these days seem to be little more than 15% efficient.
I found an option that works better for me which might be worth a try. Just sling a conventional rigid panel of maybe 200 watts UNDERNEATH the bars of your roof rack. You can still carry stuff on the rack but will lose some of the 200 watts due to shadowing. Mine still gives a good 140 watts or more in full sun.
Good luck.
Keith