I bought one of the 10w panels from Amazon which clips directly to the terminals but as my car doesn't get direct sunlight it was struggling to make a difference with my battery drain, in direct sunlight the voltage increase was more noticeable but I was trying to save a battery which had been jumped for short journeys and hadn't had a chance to fully recharge.
With a 10w panel I was getting about 4w at 18v according to a multimeter with the panel suckered to the roof. No problems with rain or frost leaving it on there.