I'm a fanatic about checking for rust anywhere and keeping the chassis well coated.
The driver side front door plastic sill was broken, so I took it off and the seat bottom, to both repair the sill and clean the carpet, and found surface rust on a weld holding the seat to the floor!!!
19 years later, and find rust on the inside!! It's been completely stripped down to metal and etching primer used, going to cover with POR-15, then reassemble everything.

 

classic location for interior rust.....usually around the bolts at seat feet because of sunroof leak (edit: or ac pan overflow or door jamb leak)...

avoid POR...it's just a plastic coating..treat the rust with something like rustmort

 

If after 19 years thats all the rust you have your doing Very well, I have a 1999, D2 and in the last few years have needed to do a few repaires including a 1/4 rear chassis, but I still drive it most day and it still brings a smile to my face when I do as I still find it a joy to drive

 

Look under the brake master cylinder both sides of the front wheel weels. It’s a stop that usually rust and usually missed