I had this happen to me, and unfortunately for me I didn't have any warranty on the vehicle either.

I have had my repaired by an independent garage for a fraction of the cost a main dealer would charge.

Basically the failure is down to a weakness in the linkage between the front and rear glass panes. The rear pane is not powered instead it is moved by a linkage to the front glass panel. When I had mine repaired I had both the front and rear linkages replaced just to be safe. The rear linkage kit cost me £60 and the front kit £140 plus fitting which took about 6 hours in total to do.