Hi I have a c max and have used thule roof bars successfully for a nubre of years. However the threads have rusted where the bolts fit to the roof. What can I do to reslve this an who is likely to be able to do it. I went to the dealer and they didnt really seem to know. Someone suggested fitting a plate over and screwing into that but I wasnt sure about the load bearing capacity. Any suggestions appreaciated.