we've had the same problem with our rear shade & 2 others. Had the dealer re-string 2 of them 1 is ok now but the other(the large rear)was strung wrong. This was last fall! Dealer ordered 2 new shades from Crossroads & we are still waiting for them!!
The reason they fray & break is from too much tension on the strings.
There are sites that describe how to re-string... the smaller ones I would try the larger ones no.
Hope this helps
2007 chevy crewcab D/A
2009 Cruiser 30sk