Just updating on this one as I'm tackling a similar rusty fuel filler pipe on my project car. After looking under my 2002 RX yesterday I can see I'll probably have to repeat the procedure there too in a few MOT's time..
Doesn't seem to be a design flaw so much as just what happens when you shower metal with water, salt, air and debris for years!
Bear in mind these pics show parts from an EP82 Toyota Starlet so the RX will be bigger/heavier but the same basic process when I get round to attacking it.
So it's just a case of inspecting from below at the back, aim for the nearside/passenger side rear area:
In this case the tank and metal lines are in quite a state too, most likely due to the car living on Scottish roads for so long. Weather conditions and road salt are a major factor.
On this car it was easier to grab a fresher (but till slightly rusty) tank, lines and filler although they are still available from Toyota. I only want to do this job once on each car so best to protect it all in the one shot:
One trip to the soda blaster and after powder costing they look a lot more robust:
Fitting it all this week. On such a small car it's a fairly basic approach, few 12mm bolts hold the tank in plus an assortment of pipes and clips to undo. The RX looks to be a bigger ballache of a job. From what I can see the filler runs quite tight between the rear subframe and the floor so that's going to require a ramp and some manual assistance if it can't be manoeuvred out past it.