[racer]

Drill out rivets and remove the two bolt/screws from the bottom and examine.

What tends to happen is that the rear wheels throw a lot of dirt and moisture up against the backside of the jackports. Then the rust begins!

I made it a habit to remove the outer panels at least "once per season" to remove the small amounts of tire debris the car attracted at track events. Driving on sandy roads is another reason. Sometimes just removing the bottom bolts is enough to "shake" out the panel and watch the junk fall out.