From the sound of it, you have rot behind the glovebox... You'll need to strip out the glovebox plastics and carpet to get a proper look at it. You may be lucky and be able to weld it with the loom in place, but you also might not be so lucky...
If you jack the passenger side up and take the front wheel off, you should see the rust from the other side in the wheel arch right in the top corner to give you an indication of how bad the rust might be.
If its extreme, then your only way to sort is to remove the whole loom as well as strip out the dash etc to get to it for welding.
Best thing would be to get pictures of it with just the glovebox removed and the carpet peeled back, aswell as a picture from inside the engine bay and under the wheel arch area like I mentioned before and someone should be able to tell if you if you are gonna get away with cutting it out with everything in place.
Welcome to the joy of owning a E30!
