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.
check fuses and for vac leak at unit and vac pump
A possible leak
If a water pump is bad they usually leak coolant
you fix the leak
waterpump or radiator hose
one way is the headgasket is blown oil can leak into the water jackets and get wixed with the oil
water pump or thermostat housing crack
Sounds like either an intake manifold or a vacuum leak
have someone sit in car and spray it down with a garden hose to find leak
Sounds like either an intake manifold or a vacuum leak
by getting a Bars Leakradiator leak at walmart
transmission leak in reverse only