Unfortunately the Megane window control isn't by a simple relay as Good Luck guesses below.
Do a simple Google search for " megane convertible rear windows" and you should see a slew of results from people with similar problem.
Here's one useful looking result:
http://www.meganeownersclub.co.uk/forum/viewtopic.php?t=12466
http://www.renaultforums.co.uk/17-electronics/98008-megane-convertible-rear-electric-window-wont-work-switch.html
You MAY get some joy by getting the control module off the regulator, opening it up, drying it off thoroughly (think in a warm place with some dessicant), maybe a squirt of WD40 and reassembling - maybe using some silicone sealant.
There are also aftermarket modules available which just give you the basic functions - these are only about £20 per window and may save some heartache.
XXX
January 2017