For the corrosion, I would treat it as best you can, then leave well alone. The car has survived for upwards of twenty years, and to be honest, unless its a minter, its not got another twenty years left in it. Mine has absolutely no rust whatsoever, but I am not expecting it to last much more than another 7, or maybe 10 years.
As for refitting the screen, use silicon grease, as used for plumbing, or vaseline. These will not cause anymore corrosion...even protect it to an extent. Do not use washing up liquid. It kills cars.