The underseal on the bottom of the car traps water between it and the metal, and they rust away merrily unseen. They have a complex underbody (dirt and water traps) with many bungs (apparently the bungs cover holes that were used on the production line to move the car but I’m not sure it’s true) this allows water in too.
The paint is poor quality and when it chips it allows water in, and it rusts.
They leak too I reckon, which rots the floors out too.
I’ve had four of them over the years, starting 20 years ago, when they were young. Each one has rusted in the same places. The last one, which I sold last year, was a particular disaster of a car. There are many components that are currently unavailable, from rear side window glass to roof ECU and comfort relays. This makes ownership a daunting proposition, and expensive too when they go wrong.
In the U.K. these are not expensive cars, a 220 with over 200k will not be £10k plus, my 320 with 125k was less than £10k and was in decent condition with no faults. Things may change, but currently these are cars for those who feel lucky or have deep pockets.
These were my favourite cars of all time, Im glad I owned them, but for now I’m sticking to the older stuff.