Did the rotted drain rubber cause the rot in the fender under the cap? Thank you, tom

Showing rotten rubber

 

I'm not certain, but it seems the front of the wheel arch is too far forward to be affected by water ingress from the filler area. Although it could be possible for water to route inside the car to the front of the arch.
I would pull the trunk carpeting and investigate from there.

 

Thank you, I always forget about the trunk! Good idea, I will look and let you know if your hunch was right.
All the Best, Tom
p.s. The filler cap drain is sus because that is the only rot around the entire car. I just got the car, 200K miles, and surprisingly clean underneath.

 

G'day Tom,

Both rear wheel arches on my car rusted out in the same area that yours did and the fuel filler is rust free and there isn't one on the other side of the car. It is a common problem with these cars as they get older.

I think the rust is caused by condensation build up on the inner skin of the car body and poor ventilation in the wheel arch area.

I found some repair panels online and had a local body shop do the repair work.

Cheers,
Jeff.

 

The area in the C pillar is filled internally with foam, which seems to hold moisture with the inevitable result. I think the fuel filler drain is routed outside of the C pillar.
ps you are missing the screen which fits on top of the drain