So here we are in the great north woods , The weather is turning cold so it's time to find an indoor project .Mine this winter will be a70gt .so far I'veredone the floors striped out all of the dash ,removed all of the little furry friends and there homes.waiting to reassemble the heater not knowing if there should be alarge opening inthe body over the fan .
Do you mean right above the fan, so that the wheel well connects to the heater area? If you have a hole there, you are in good company, since it is very common for mud to collect up on top of the "shelf" inside the wheel well and rot out the sheet metal. Then the mud falls down into the fan and motor and it quits working.
If that's not what you mean, can you post a picture?
Yes. Fix it when you have the heater fan & motor and heater core out, if possible. Cut away all the rusted metal (unless that describes the whole car ) and cut a piece of sheet metal to fit. If possible, make it so it has a lip up against the outer fender (next to the engine compartment) and another lip down over the inner fender, so that it sheds water better. Attach it by either welding, pop riveting or use screws. And then seal it thoroughly so that water can't get into the fan motor.