 Volkswagen should be more than a little embarrassed.  The grade of plastic specified for use in a German designed sun visor should be UV resistant.  That it broke down and became brittle in 5 years, with less than normal use (and no abuse) is unacceptable to me as a customer, and definitely to me as an engineer (I understand mechanics of materials very well!).  I believe that a defective or knock-off part installed on my vehicle at the time of manufacture, should be replaced at no cost to me and chalked up to quality parts control.. and lets have a lessons learned here.