what i did on my [really] rusty cowl vent screws,which were smaller than those holding your windsheild frame together, was to heat up the head to almost turning a very dull red, then place a birthday candle on the screw for about 10 seconds or so. the wax melted down around the head into the threads, and the screws came right out with very little effort. just make sure you watch your fingers as the candles get short kinda fast. also, make sure the screwdriver you use is the biggest, best fitting one you have that fits the slot so you don't cam out the slot, causing you to revert to "plan b".