A probable source of water that is causing your rust is the passenger side cargo window. The cargo windows are notorious for leaking. Unclip the soft carpeted panels and look up where the glass meets the frame. Look for water trail marks running down to the wheel well and for standing moisture in the bottom of the wheel well. You can also stuff some tissues or paper towels up under the window and see if they get wet after a good rain.

If that window is leaking then it will have to be reinstalled. Sometimes glass leaks can be stopped with a liquid silicone product, but the underside of a glass is poorly located for this kind of fix (the silicone is unable to flow into the gaps, unless you turn the car upside down!). If there is rust under the window, of course that will have to be patched too or more leaks will occur.

Welding is the best remedy, but you can treat the rust with a chemical agent (Ospho, Extend, naval jelly) and fiberglass a patch over it (available from Pep Boys, etc.). If you are very meticulous with the fiberglass and also seal the patch with paint/undercoating, it can last a good long time.

Thanks for the well wishes - ditto.
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'84 '285' rice-eater with lotsa hp & performance goodies