This is a very very common problem on RST's.In fact I scrapped my shell because of this very reason.A new roof skin to get rid of the sunroof altogether would be the best idea in my opinion,but new roofskins (with or without a sunfoof opening) are not available brand new from FORD or anywhere else it would seem.Best bet is to cut one off a car down the breakers yard.Its not easy to do,as the roof is a major structural item,youre body builder will have to be very very good!!!A few people on here have got all the rust cut out of their roof,welded in new metal and then spray a f*ck load of waxoil under the roofskin,it worked for some,but the rust came back with others.Hope this is of info to you.Do a search on here as it has been talked about before over the years.All the best Micky