I can't specifically tell you how it can be done, but I took my 88 Brougham in to a car detailer/upholsterer to have my vinyl roof redone and he knew right away what the problem was. That was two years ago and only now have I noticed the water starting to come in again so I will be taking it back. It had something to do with a clogged drain channel I think. I had the same problem with an 89 Sedan Deville. Sorry I can't be of more assistance, but at least you now know that the problem can be fixed.